Since our feedback has been solicited, I'll provide it with honor:
-Shorten the gap between successfully casting a spontaenous spell and successfully casting a formulaic spell. The incentive for taking the time to learn a formulaic spell could perhaps be measured in botch dice: Casting a spontaneous spell could give +1 botch die/magnitude if a zero is rolled. You CAN cast a spont spell in dire need, and your total is just as good as a formulaic spell--but woe betide you on that 1-in-10 chance you threaten a botch.
-Have advancement be more balanced between seasons and quick adventures. Allow more incentives for magi to go on a quest, provide less pressure for magi to slam their sanctum doors shut when a quest beckons. As it stands now, the only benefits to going out are getting loot or solving a problem. Maybe award adventure XP in addition to study XP.
-Make it easier to learn/study magic outside current experience, such as a magic item or a non-hermetic effect.
-Better guidelines on some of the Arts, especially Muto. (Take it as a compliment that I higlight Muto in particular, it means I think you did an excellent job with the other 14 Arts).
-Better ways to make a 'type' of mage, as opposed to an art specialist. For example, guidelines and opportunities to make a necromancer, summoner, warder, beast mage, etc. There are some good options already existing, just listing an example for some useful 'how-to's.
-A more intuitive, streamlined combat section.
-Less 'meta-useful' Arts. This one is harder to explain. What I mean is, some Arts are incredibly useful in addition to their spells, some less so. For example, Creo, Intelligo, Vim, and Corpus are used in many different totals for all types of Magi, including creating vis, making longevity rituals, investigating lab enchantments, etc. Arts like Muto or Aquam, significantly less so--they do not factor into any important totals.
This is all I have for the moment. Again, I compliment you on Ars Magica. I first played it in 3rd ed many years ago, but only for a session or two. I ran a 4th ed saga from start to finish. We all enjoyed it even though we had some arguments about the rules. 5th ed was magnificent: I ran a saga and I hope to start another in a couple months, once I move back closer to home and my friends.