So,
I always find yair's attempts to tone down Ars Magica interesting, but players who really want a low magic setting are probably looking toward something like Pendragon or Harn. I also notice that these attempts make certain niches way too goodcompared to the others, even more so than the standard rules. (for example, the classic "treat all arts as abilities" makes any build centered around magic theory or around life linked spontaneous magic much better by comparison than it was before.)
Rather than considering affinities and magical foci virtues that need to be toned down, I consider these to be the cornerstones of consistency. That is, having an affinity with an art or ability is a cornerstone minor virtue. Having a special ability in some area of magic that is less than a technique plus form combinationthat doubles the lowest art is also a cornerstone minor virtue. I don't consider either of these to be broken. Good, yes.
I consider the ability to aim spells to be broken. This is because it makes magic resistance useless, and Parma useless, which is against the founding fluff of the game. also, the ability to boost casting totals with vis can puncture anything. But this can be fixed: a supernatural being has a base magic resistance equal to the magnitude of its might Roundup. This adds to his soak and to his resistance roll against anything supernatural. He also has the usual magic resistance equal to 10 times his base magic resistance. Parma is only obtainable through initiation, and the only script is offset by a major oath: specifically, the hermetic oath properly administered. Coming up with a new script should be considered a hermetic breakthrough. A wizard who has Parma has a base magic resistance that is equal to his warping score, plus his score in the appropriate form, which works exactly as a supernatural beings based magic resistance. A wizard with Parma has the usual magic resistance equal to 10 times is based magic resistance. I would prefer finesse to go away entirely as an ability, but at the very least any spell or supernatural ability that is modified by finesse can be resisted. Whenever there is a doubt about whether magic resistance applies, it applies if the player ( or the Angel who exists to look out for that character's well-being) wants it to. Vis can no longer be used to boost totals. Charged items have a maximum level equal to half the lab total (If a lab text is used 1 charge is created per magnitude of the full lab total as usual), and adding penetration to any item is on a one-for-one basis.
Good defenses help a lot.
I consider teleportation spells to be broken because they simply don't fit. Summoning a spirit is a little bit different, because such things become immanent through various means including the appropriate spell. also, traversing a regio boundary is also different because this too is not physical movement. Otherwise, any magic that changes an object location must do so by moving it through the intervening space. this is surprisingly helpful.
Finally, there are a whole bunch of spells that break the rules, and I consider these broken not because they are powerful but because they break basic principles. For example, the broomstick spell (I forget the name but it's the one that lets you fly on the broomstick) is utterly broken because everyone uses it as a ReCo spell even though it is actually only able to move a plant object, requiring the user to hang on for dear life as this thing he is trying to hold suddenly lurches up into the air at an accelerated speed, with nothing intrinsic to the spell helping him move or maintain balance( I don't normally care about physics or anything like that, but I do think that a ReHe effect is not a ReCo effect.) Of course, some of these spells are very powerful, as well as breaking expectations and rules, such as some in the Guernicus section...
If you want to knock power down a tad, there is absolutely nothing wrong with simply getting rid of the magic theory ability. All it does is boost lab totals and clutter a character sheet. I don't think the game is distorted by this simplification. and it does reduce power a little bit, although characters are no longer limited by magic theory for how much vis they can spend in the lab in the season.
It is probably also helpful to rule that arcane connections must be meaningful, not a scrap of garment or fallen hair or anything like that. This makes getting one a more worthy story, and defending less sordid than paranoid waste management.
Anyway,
Ken