I think Ars Magica 5 is almost too complex for play. I mean, it's great for advancing, but in play, say combat, there are a lot of fiddly bits. Fiddly bits slow things down, so combat tends to be avoided or glossed over. Not that there's anything wrong with that, but a lot of players do enjoy combat. Combat can be easily avoided when magi are overly powerful, which they can easily be.
Focusing on fiddly bits for weapons combat involves the player and the SG comparing totals, and if the attack total is greater than 0, a subsequent calculation happens to compute damage, which is then compared to soak to derive total damage, which must be looked up on a chart, because of how wounds are applied in Ars. Broken down into components, a single attack has: attack roll, addition of bonus, defense roll, addition of bonus, comparison, (if attack>defense) derived damage total based on weapon, compared to soak, (if derived damage>soak) final damage, application of final damage and determining subsequent penalties. Each successful attack has 10 actions associated with it, and one of those actions is conducted by a secondary party. Other game systems have a single roll for an attack against a static defense, and a subsequent roll for damage if the attack overcame defense. Once defense is known, the player can make both rolls and report the results at the same time, with the GM only prompting the player for his action.
Hermetic Magical combat has fiddly bits, too. Casting total, which is a total computed from at least 3 static components (te, fo, sta), 1 environmental component (aura) and a random die roll component, along with optional components such as spell mastery score. There might be other components of the total, but that's the basic total,and adding anything else adds even more complexity, which only reinforces the idea that this is fiddly... Casting total is then used to compute penetration, which we will keep simple as Casting Total - Spell level + Penetration ability + Mastery, if applicable. Penetration is then compared against MR. If there is penetration, then the effect is applied, we'll assume it's damage. Damage is rolled, soak is rolled, compared, and if damage>soak it moves to the application of damage and determining subsequent penalties. One of the odd effects, is that it becomes tiresome for the SG to track the aura strength, so it's a lot simpler to tell the players what it is so they can do the entire computation on their own, and it often happens for magic resistance, as well, which invalidates a heck of a lot of Intellego magic. But once MR is known, players can make both rolls for casting/penetration and damage at the same time and have the results ready to go when their turn comes back around.
Then you have the opponent side of the combat, which has all of the combat stuff above. Magical powers still have to check for penetration. Round 1 is done and I'm tired of it already.
So, that all leads into my wish: on screen action needs to be simplified and refined to minimize the computational needs of the players. Make off-screen as complicated as the player/troupe can tolerate, but please simplify combat.