The Vis thing would actually be pretty cool... Though the Redcap Vis Trade would go out of business so fast if it wasn't, in fact, essential to people performing their preferred magical activities.
LuckyMage mostly summed up the reasoning behind my proposal.
The fact it wasn't proposed as an actual serious House Rule nonwithstanding, they might still be more powerful than a D&D wizard in relative terms. What I mean is that, vis-a-vis (pun intended), they are indeed overwhermingly less powerful than a D&D wizard. But power is something that is measured as compared to your environment, and that HR would still leave them way more powerful than their mundane environment in one of the low-fantasy versions of canon, while D&D wizards live in an almost universally high-high-fantasy world. Thus, and assuming you chose a very low-fantasy version of Mythic Europe, while they'd be less powerful than a D&D wizard in absolute terms, they'd be way more powerful than their mundane environment, radically more so than your average D&D wizard, which is only marginally over-par relatively to their game world.
But this was still not meant to be a playable House Rule, just part of the buff/nerf game. 
It's more of a challenge to have house rules that still let the game look like Ars and be playable. If there aren't any limits or boundaries in this "game" then it isn't really a game. Many simple house rules can be made that change the game but it's still Ars Magica. Making spell casting take 15 minutes doesn't do that. No one would play it, as a Mage.
Though I see your point, and I mostly agree that it wouldn't look quite like Ars Magica, I don't fully agree that it wouldn't be playable.
Granted, magi wouldn't wield the enormous amount of immediate firepower they have in Ars Magica, but if you did keep the world really really low-fantasy, they'd still be the most powerful people around. Magic would definitely be useless in combat, but for plenty other situations I don't think it would change things too much. It doesn't really matter if it takes one round or three hours to cast the spell that would enthrall the king's mind to your will, for example.
I admit that durations would maybe need to be adjusted, since short-lasting spells would not make much sense anymore, and maybe range would need to be considered only at the final moment of casting the spell, when activating the effect. But I do think you could run a very enjoyable game that felt more gritty and "realistic", but where PCs would still be the ones calling the shots when it comes to interference with mundanes. The only real thing you would need to do is focus it almost entirely on the mundane world, and leave everything else as subtle and very rare.
Now, I'm not claiming it would be the same kind of game as a normal Ars Magica game, of course. I'm not even claiming it would really be Ars Magica. I just think, personally, that it could still be quite an enjoyable game to play a magus in.
Well, there is a restriction that will tend to keep the game looking like a distorted Ars Magica: Namely that the rule should interact with as few other rules as possible. And given that almost half the core rules would need rewriting if this HR were to be turned into an actual part of the game, well... The fact that breaking the game is a good indicator of success doesn't mean he just won for breaking it the most. 
Though just to add a bit of hilarity to this situation, an otherwise effectively built magus can be worse than this character in an entirely regular game, since there is a flaw that removes your ability to cast Rituals and makes you cast Formulaic spells ritually (can't remember the name, Serf's Parma) as well as the Weak/Difficult Spontaneous combo to remove Spontaneous casting... So your Formulaic spells require vis along with taking forever, and you get arguably even less spontaneity than being able to spontaneously cast over the course of an hour. 
I made made a character with Rigid Magic and Strictured Caster. He is playable because he can make stuff and do lab work. IMO flaws that limit Spontaneous magic aren't a big deal. Limiting formulaic is huge, though.
Advance experience / lab work / etc. on a monthly basis, rather than seasonal.
So basically, the massive increase to the casting time of spells would make for very Gandalf-y magi who mostly rely on enchanted items and high Realm Lore and practical skills to get by. Definitely a very different game, not really keeping with Mythic European fluff, but it wouldn't necessarily be a bad game.
@TimOB: Huh, interesting, interesting. Makes everyone a good deal more powerful... Except Learned Magicians. Poor guys
Here's another Simple but Huge HR.
Treat Aura ratings as magnitude, using the magnitude in the lab total calculations as normal, but multiplying the magnitude by some factor, say 3 or 5 for determinging the bonuses to MR and spell casting. I'll use 5 for my examples. For the first example, a Divine Aura 3 would be a 3x3x5 or -45 to spell casting totals. A magic aura of 5 would add +25 to spell casting totals, which puts all 1st magnitude effects into the realm of even an apprentice with newly opened Arts. It makes a high aura truly "dangerous" as is mentioned in flavor text in a couple of places. A magnitude 7 aura, like Durenmar's, would make the bonus to casting +35 and some unwise apprentice could attempt conjuring the biggest fire he could manage, which would be bad...
Ooooh.... Nice one, JL! That makes home-field advantage a MUCH bigger deal, and simultaneously explodes and implodes character power based on where they are. And it really only affects the Aura and MR/Penetration rules, rather than the entire book... Definitely some fluff changes, but that's only to be expected. Again, nice.
Obviously, fluff will be changed by any HR that is Simple but Huge. Otherwise, it wouldn't be Huge. 
I assume when you mean that for it to be Simple it need to affect as little else as possible, you mean "rules-wise", not "fluff-wise".
The version of this that I've considered (posted?) is that any compact vis, either distilled from an aura or concentrated from its original source, is generic vis, usable in any magical activity. However, any vis kept in its original source form counts double for activities very closely related to it. So the antlers of the great Moose of Moosecovy can be used for anything, but count as double when used for whatever those antlers correspond to (masculinity? presence? authority? getting trapped in thickets?), as long as the vis is kept as those antlers. Covering an entire Art is usually too big, except perhaps for vis from an elemental or something that iconically represents an entire Art.
Either change is simple but not huge, imo.
There is no such thing as raw vis. Specific items obtained in stories (such as the antlers of a magical stag) simply provide a fairly large bonus to lab total when enchanting it (and a smaller bonus if it is used up when enchanting something else) and/or a casting bonus to specially designed spells (much like Potent Magic does).
EDIT: Rituals are either anchored to a magical aura, or need to be recast periodically to remain in effect. Healing rituals need only last as long as natural healing would have required, and stat-boosting rituals simply don't exist.
@TimOB I like that one. Monthly advancement results in approximately 3x the power results most of the time.
A simple enough one is replace stress die with exploding die.
Roll the dice. On a 1, check for botches (additional 1's on the botch dice mean botches). On a 0, roll again and add 10. If it's another 0, repeat, stacking the 10's till something not a 10 is rolled. Reduces the exponential explosion that a stress die has while still allowing it to explode.
Edit: A less excessive version of JL's suggestion is also pretty nifty.
Double aura values for totals.
So a 1st level aura of magic is +2, a 9th level is +18.
A 1st level divine aura is -6, a 9th (The middle of the Vatican, for example) is -27.
The general idea was to exploit the idea of "home field advantage" à la the home field advantage of Harry Dresden in the Dresden Files novels. Harry is strong enough to do some things at home while unable to do some things elsewhere. In the novels this is ascribed to the theory of the threshold or home. In Ars Magica this creates a dynamic which makes Companions and grogs even more important.
It also makes the idea of being able to cast certain high level spells, like IoL or BoAF at home possible and reasonable for a lot of magi, while doing them elsewhere might well make one tired rather quickly. It alsoseparates the idea of what one knows from what one can do into something more concrete. In Ars ruleset, one can almost always do what one knows, which I find frustrating...
Edit: I suggested 3 or 5 a something consistent with the overall Ars paradigm, but any factor greater than 1 would work. I like 5 because it highlights the disparity a lot...
If you want to make it a truely home field advantage, have it only apply once a character has lived within that aura for X amount of time, otherwise it's the usual Am5 rules.
So just finding a magic aura of 3 is worth +3 to all casting totals when you're in it. Live there for (let's say) a year, and it's attuned to you, or you to it, so it's now worth +15.
True home field advantage, still some bonus working from other mystically appropriate areas.
Means assaulting a hostile covenant is STILL hard work.
If you consider the Aegis as part of the homefield advantage it becomes a much different issue. People often forget that the RAW has the aura bonus adding to the MR. When you calculate that an aura of 5 adds 15 or 25 to the MR, while the Aegis subtracts half it's level from the CT, you quickly run into situations where magi at home, within their Aegis are really powerful, even against other magi.
The idea was to let the magic aura always be home field advantage, to some extent. And when you go outside of that, powers that you could normally rely upon just don't always work. The BoAF that you can always cast without fatigue at home becomes problematic when you go out into the "real world" and work it even among mundanes. In most Ars games that I've been part of, casting a spell is generally consider fait accompli. Very few magi that I've seen played or have played myself have spells that they know, but can't cast reliably. I like the idea that they can cast them reliably at home, but can't cast them reliably elsewhere and also have significantly less power outside of their homes. Granted, a divine Aura 3 is -45 or -75, but the idea is that these aura would be much less prevalent, and an aura 3 is about the highest divine aura one would ever encounter unless they were seeking it out. Cities, as a rule would be Divine 1, with Divine 3 being closes to the church(es), and two churches really close together might push it to a Divine 4.
Faerie aura are interesting, because magi would get the traditional bonus, and it would make a Faerie Aura 5 useful: 5x5*.5=+13 bonus. I've honestly wondered why magi would ever settle in a faerie aura, as PCs, sometimes it's the only choice, but creating an organic setting, with plentiful aura around, I have to wonder why one would go to a faerie aura... Faerie Magic, not withstanding.
Hi,
I had a version of the Aegis home field advantage: Every year the Aegis is performed over the same area, the area's Aegis score gains 1xp if the magnitude of the Aegis ritual is greater than the score. Every year in which no Aegis ritual is performed over the area, the Aegis loses 1xp from the score. An area's Aegis score adds to its effective magic aura, but only for participants in that year's Aegis ritual. Being granted a token is not sufficient. An aegis ritual that is both larger than the area and totally subsumes it always prevents the loss of xp, but only add xp to the score of regions with the least number of xps. Thus, if the entire area has the same score, the entire area gets an xp.
Being part of the aegis ritual is therefore an even bigger deal, and there is real benefit to to inheriting or joining an older covenant.
Oh, and because rules should have weird corner cases: A Living Ghost bound to the exact Boundary over which an Aegis is performed gains 1xp per magnitude of the Aegis ritual, and is always considered to be included, even if the actual celebrants do not know about the ghost or actively oppose it. (Although magi opposed to the ghost can simply choose a larger or smaller Boundary... or just choose PeVi.)
Anyway,
Ken
Here are a few. I think they work well together but can be used separately, I suppose.
First the rules, then a conversation.
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All PCs have Confidence, including PC grogs.
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A confidence point allows declaring the result of a die roll to be any number from 0 to 10; no botch dice will occur but choosing a 1 for a stress roll does allow further die rolls, each and any of which can be affected by Confidence. This can be done for any activity or action. Aging is not an activity, and the experimentation tables represent factors the character cannot account for, so confidence cannot be used for these. This replaces the usual uses of Confidence.
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Grogs do not receive xps and cannot advance Abilities. They get a flat bonus to all totals involving an Ability equal to (age/10, rounded down.) Optionally, they get a flat penalty to such totals equal to their Decrepitude.
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A grog begins every story with points of Confidence equal to his score, and can gain more during a story. All points are lost at the end of the story.
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Confidence can be gained in the usual way. A PC can choose to spend 1 xp gained from a season of adventuring to gain confidence points equal to his Confidence Score. This benefit replaces the vaguely stated benefit of Self-Confident.
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The standard grog rules can be used for standard grogs. It is also permissible for the SG to create grogs that have more (or fewer) than usual xps and more (or fewer) than the usual allotment of virtues. For example, a baron and retinue of highly capable knights can be built as grogs.
Comments:
These rules are intended to work together. Because grog advancement is simplified, grogs are easy to maintain and easier to create. Because grogs of certain kinds are not limited to the same number of xps, very different kinds of grog can be created, similar to the Spring/Summer/Autumn/Winter types of Magic Being. Because grogs have Confidence, they get to shine when needed during a story. Because Confidence is better, they really will shine when Confidence is used, and will be able to justify being brought along (sure, grogs aren't as good as magi or companions, but they bring along important clutch successes.) Other characters also get greater control over the random number gods.
These rules damage the usual assumption that xps are tightly correlated to age. Grogs statted with the usual number of xp will not advance at the same rate or in the same way as grogs using the normal rules. Grogs that begin play as youngsters do not work well under these rules; it might make sense to grant blocs of xps periodically.
Any
Our troupe has grogs with Confidence (though normal Confidence), and it works pretty well. The only difference is that we don't keep track of their Confidence Points from session to session, to reduce book-keeping. They always begin each session with 3 Confidence Points, no matter how many or how little they had at the end of the last session.