GURPS is worst indeed...
I reckon Ars Magica system is complex, and I like that. The problem is that complexity also means time lost during play, summing numbers. I think that it should also be easy and straightforward in play, as its goal is not (like rolemaster or GURPS) to simulate the universe but to support roleplay by defining what characters can or cannot do.
I'll second that. I don't mind the complexity of the core system, probably because I'm so used to it, but it's beyond me why we might need addons like point value systems for designing cathedrals. More storytelling, less fiddly bits, please.
You misunderstand. If you´re running almost any game or rpg that involves numbercrunching at any level, a calculator SHOULD be a standard equipment to always have present.
I've played with people who'd used a calculator to add 3 (single digit) integers, just because they couldn't be bothered.
The presence of calculators is not a problem.
Differential equations?!?
I assume it's an hyperbole, but if it isn't, I'm really curious about it!
Maximum velocity of a vehicle depends on its mass.
As the vehicle moves (and especially as it moves faster) fuel is expanded, changing the mass of the vehicle, thus changing the maximum velocity of the vehicle.
The math stops being nice around then.
That, or the solid rocket fuel vehicle 
I was confused a minute.
And the maximum velocity does not depend on its mass, but on the fraction of its mass that is propellant.
You should have said spacecraft, too.
And this has nothing to do with GURPS, any realistic space RPG would have the same issue.
Oooops, spellie 
And as for GURPS vs Realistic: yes. GURPS just happens to be the only one where I've personally encountered it.
Thanks, I was thinking everybody here prefer numbers (that we already crunch during work hours) to roleplay (that's the interest of Ars Magica for my group). 
I know this is an OOOOLD topic, but I've been playing other games quite successfully for years.
And I think the time is approaching for people to think about a new Ars Magica, maybe in a new format?
If I could redesign Ars Magica, there's a lot a throw out and a lot I'd keep.
The medieval myth and history is fun. The community/covenant building is fun. The idea of grogs vs. magi vs. companions is in keeping with the setting. Troupe play is interesting.
The character creation process is torturous for most of the players I've ever managed to recruit.
I can sell the game to new groups on "It's more true to history than D&D, but it's mythic history so YOU don't have to be accurate." But once I hit character creation, I get rage quits.
The ones that don't rage quit at the long character creation process might might make it to the first spell that gets cast.
And I'll be honest, as I get older, I like simpler, more streamlined stuff because my brain isn't what it used to be!
I bought the first edition waaaaay back when and I thought it was a cool idea. The only thing I could get anyone to play was Shadowrun or Vampire, eventually Mage, but I thought it was cool idea.
I have run a couple "hacked" versions of Ars Magic, but they were pretty heavily salted with other properties (like a Harry Potter game set during The Anarchy and a Sorceror's Crusade game where I used Hedge Traditions instead of the houses and kicked it back to the 14th century, because that group could handle buboes and class warfare.)
The era shouldn't be problematic for me, but I've had a lot more success running wizard games in Hypatia's Alexandria (because I can make players watch Agora) and the age of Charlemage/Aaron the Just of Baghdad/Empress Irene of Athens, contemporaries for a golden half decade at the end of the 8th and beginning of the 9th centuries)
I have had no luck with the traditional houses, but using Merlin, Viviane/Ninue (as Lady of the Lake), Huon of Bordeaux (or Auberich) and Semiramis and Gyges of Lydia and other famous wizards or magical collectors as masters or house founders has worked pretty good. It acts as a kind of shorthand and gives players something they can look up on their cell phones and explore.
ANY kind of broad strokes would really help.
Templates for quick starts for instance.
Using grogs as a kind of Hit Points is an idea I've done, basically making players name (just name) 5 or 6 grogs and giving them the opportunity to turn the surviving grogs into companions. It makes the danger mean more (We lost Jehan! I was going to turn him into a companion for your insane
Fafnir (Flambeau) mage! Or, Ha! Eudoxia made it! She's joining my lab staff as a herbalist!)
I still like the spell combination, but I often experiment with simply putting a range/duration/target pastiche onto each Art and Technique instead of a level.
For instance, I might have Rego (Sunrise/Sunset) and Mentem (Room) for a wizard, meaning all Rego Mentem spells this wizard cast always affected a Room and lasted until Sunrise or Sunset, whichever came next.
To make things easier, in the Hogwarts during The Anarchy game I ran, I had players assign a set number of Range/Duration types to Arts (rethemed silly things from the Potter spells) and a set number of Range/Target types to Techniques (similarly rethemed.) It was a heck of a lot faster!
Most of my players want a little customization but not a lot. One player would like to be able to really get in there and play with the numbers beforehand, but he's the exception rather than the rule and there's no reason you couldn't put out the basics first and then the fiddly stuff later.
I would also go with the more interesting ranges/durations/targets
Start players with a handful of evocative limiters like Watercourse, Crossroads, Incense, Ceremony and Year and a Day then let players develop more generic ones (Circle, Diameter, Voice) through breakthroughs.
PS I always called the Voice range "Echo" because it carried the idea further. Get it?
Oh, here we go.
This is where I owe an apology:
There actually ARE templates in 5th edition Ars Magica. One for each of the Houses
But I skimmed over them for years assuming they were NPC templates.
Since I never got much past character creation in an Ars Magica campaign, I never really looked at them. Whoops.
I think it's because I have never liked the Houses.
Long story short:
I bought the first addition on a whim back when I was in college. It was in the discount bin of my local hobby store behind some magazines on tanks. The first word I read in it was "analgesic" or "analgaesic" for a longevity potion. (Wait, no, it was antagathic.)
But by the time 5th Edition came out, I didn't have time to read game books cover-to-cover anymore. I skim a lot.
I agree 100% Timothy
As much as I got into detailed libraries in 4th ed, with the stuff from WGRE: Physical Quality, Glossing, damage to books etc. I'm now a firm supporter of "Single Digit Libraries", but without a good solution.
For Arts I could maybe live with "Full Level Advancement" so you don't need to track 'loose' exp. However my beef is with how this may prevent spending Story Exp this way, and how it might much up Vis Study. Vis Study is already suboptimal when compared to Book Study, and if you don't keep the exp rolled when not sufficient to increase a level, it sucks.
I'd be happy to compare ideas and thoughts.
I never worried about story xp being restricted to abilities in 3ed. I quite liked that magi had to do both stories and study to advance all they needed. Therefore I also liked it that nobody had thought of books for skills and talents.
The 4ed book rules were a great step forward because an individual book became a treasure, to be traded/looted/bargained for, individually. Even though, I also agree, that it came with a lot of mechanics which does not help that cause at all. If that mechanics is taken to imply that the magi would and should always be able to squeeze out the last quality point, it becomes a nightmare, but with a bit of moderation it is satisfactory.
But yes, Timothy has a point that when people study, they do not study just one book. (They do not study just one ability either, but let's leave that out for now.) What really annoys me in the 4/5ed book rules is not the book keeping, but the fact that the many Q3/4/5 books, even books of great r/l significance, are worthless. In reality, such books provide important ideas which the advanced learner needs to study, not for a season, but to look up and study and compare to other sources. A system where all these books can contribute to the library's overall score would be swell. Then each book remains a treasure, but it works best in a larger library. That would be the most important consideration for a 6ed IMO.