I didn't take it that way. It would be helpful if you could expound upon your specific difficulties with the game, than to generalize, though. You have to understand that some flaws will utterly kill a concept, and that they aren't necessarily equal. It would probably be a bad idea for a combat focused magus to take Slow Caster or Disorienting Magic. It probably doesn't hurt them too much from taking Rigid Magic, Chaotic Magic, Difficult Spontaneous Magic or Weak Spontaneous Magic. The concept is going to be key to determining which virtues and flaws you select. If you have a magus that doesn't want to throw-down and has several shield grogs at his disposal, then Slow Caster or Disorienting Magic isn't so bad.
Ars Magica supports a fair range of concepts, but those for Hermetic Magi are naturally constrained by the Limits of Hermetic magic. This is a valid criticism, but I personally think the game benefits from the boundaries imposed because they help give the system a greater sense of structure. That said, they are not hard and fast rules and can be circumvented in a number of ways (Mysteries, Original Research, Integration, and Hedge Magic).
In answer to your question about the realms, I believe it's a reference to the facts that:
A. A creature can only possess a Might Score affiliated with a single realm.
B. That powers from each realm are considered separately - for example, possession of The Gift allows a magus to learn supernatural abilities associated with the Magic realm, but wouldn't be able to learn Divine, Faerie, or Infernal powers without True Faith, Sympathy, or a single point of Infernal Warping, respectively (or taking the powers independently as virtues).
I do not think your comments sounded offensive, at all. I would say this.
Ars magica does not offer a system meant to be "universal": there are lots of magus concepts one can come up with, and that do not work with the rules of Ars Magica as written. Many other games out there do universality better (e.g. Wild Talents). At the same time, from my experience, universality comes at the price of requiring a lot of Storyguide discretion in adjudicating stuff.
In this sense then, Ars Magica is on the "solid, highly playtested mechanics that require little adjudication" side of the spectrum, like D&D. I can't think of any game that definitely beats Ars Magica on this metric, though many are comparable. As I said, games on this side of the spectrum tend to present a relatively limited palette of character concepts. I think that Ars Magica provides a much wider palette of (mage) concepts than the typical game on the same side of the spectrum: even though Ars Magica cannot make every possible mage character concept work, it can make more mage character concepts work than any other game I can think of with the same level of solidity.
I hope that helps!
If I sounded like I was attacking (and in re-reading, I may have), I am the one who should apologize - that was not my intent. But nor am I the type to pat someone on the head and say "Of course" when I think they're wrong at face value (ask anyone here 
).
I do not believe your comments were out of malice, but out of ignorance. And before that sounds like an attack, it's not - we are all ignorant of new things, it's impossible not to be. It's understandable. But pronouncing judgements when we are ignorant... that's a recipe for stepping in it.
The "or far worse" comment was intended in that direction - your judgement sounded prematurely and casually dismissive, and the comparison to D&D superficial to the point of making you, yourself, sound suddenly less astute than your previous posts have indicated. In short, perhaps... "fatally over-simplified" might be a good, neutral description?
Does AM have limits? Absolutely. Do those limits make it comparable to a 2nd gen RPG magic/chargen system? Not remotely, imo, and it would be a mistake on several levels to suggest such. ymmv.
Naturally, in the same way that dumping certain stats in D&D can be semi-fatal.
Flaws (Hermetic or otherwise) are not all meant to "balance" Virtues, despite the +/- math. Many are offered to create a more colorful story, and some are cheap at the price, depending on the concept. As far as Major Hermetic Flaws, some magi wouldn't blink twice at Blatant Gift, or one Deficient Technique, or Weak Magic Resistance, or a Restriction, while other concepts would, indeed, be crippled by such (or Players blanch at same - ymmv).
The trick (if you want to call it that) is to either find Flaws that do not contradict the strengths/interests of your concept, or ones that challenge those without overly interfering with them. Very doable, with few exceptions.
I agree with this. I think there is a second disadvantage to universal systems, as well. If you look at a system like Mutants and Masterminds, which is very good at universality, there is no difference between a wizard and someone with a technological utility pack, beyond the game mechanical effects of their powers. This makes things a bit bland for my tastes, although if you are doing superheroes it is clearly the only way to go.
Ars Magica has rules that differ between kinds of magician, making them feel very different in play, but also limiting each type. If you only have the core book, you only have the core type of wizard.
This is also one of our aims. It's nice to see that someone else agrees that we are achieving it.
From a beginners perspective Ars is far more daunting that DnD, and that I think is why the system is elegant and also very complex; justifiably so on both counts. To come at the game from DnD you'll need to think about the game as a shared storytelling session, not an RPG.
Balance only exists marginally in the context of the story; e.g. the game make no attempt to force a low level spell to only have low level ramifications. There is also no balance between the combative power of a grunt fighter vs a fledgling wizard; the wizard should win almost all the time. The setting & lore is the controlling factor in most games I have played, meaning that while a player can conceive almost any action, the ramifications in-play are what keep them from doing the edge case scenarios, or crazy things.
I wouldn't necessarily agree with this, but I only play a few systems- Ars MAgica, Shadowrun, and GURPS. I think solid highly playtested and requiring little adjudication would apply to all 3 compared to the industry though AM would, I think, come in at the bottom of the three on these criteria.
On the other hand, I don't think the feel of magic in Ars Magica could be accomplished with the rules as clear cut and "processed" as they are for either of the other two games. The character ranges are narrower in some ways (there will never be a hacker in ars magica) but there is far more depth to the magic system and while using similar mechanics gets more feel of variety than GURPs manages with multiple magic systems.
I'm picking spells and I just got to the Corpus section, and I am wondering if I flubbed up big time. My character is Giant-Blooded, as per the Major Virtue, and the Corpus section mentions that a human is considered applicable for the Individual target at size +1 or smaller. Will this have very vast negative repercussions, or will the necessary change in spell magnitude actually not be much trouble?
Negative repercussions? That depends. If you expect to have a lot of Corpus spells cast upon you, by yourself, or others, then yes. You need to work with the SG to account for this, and add a +1 size modifier so that your spells can affect you. On the other hand, any enemies who cast corpus effects upon you will likely have them fail due to your sheer size.
Whether the change in spell magnitude causes trouble is a function of your Corpus lab score for each technique involved. You need to up it by 5. Means Wizard's Leap, for example is now a 20th level spell...
The thing to remember about many of the Major (and Minor) Hermetic Flaws is that they can be completely ignored simply by using enchanted devices (bought from other magi if necessary). Your Restriction prevents you from casting spells on blue eyed people? Use your high penetration wand of Pilum of Fire. Magic Addiction? Doesn't apply to enchanted devices. So while the Hermetic Flaws might seem worse than the Virtues, the means to mitigate some of their problems exists.
Personally, I find that most of the general Minor Flaws are way worse than their points costs suggest.
For some spells, Part Target will suffice. For spells cast on others, use of Individual is fine, you just can't apply them to yourself. For stuff on yourself, yes, you'll need +1M for Size. BUT, you get basically blanket immunity to a lot of other people's Corpus magic. The trade-off is well worth it.
That sums it up well.
Note that "the mind" has no size, so you're "normal" there for Mentem.
Also, Target:Group can divide the sum of the affect into any smaller units desired, so while your mage might count as "2 Individuals", nothing special is needed beyond that consideration (see insert "Targets & Sizes", p 113, col ii top).