For me, there are many high-value rules, but probably my favorite is Visions. That Story Flaw just rocks.
The lab experimentation rules are a close second. "Hey, I want to invent Shape of the Woodland Prowler in two seasons instead of three! I'll experiment!" Then, "Finished! Wait a minute - what do you mean I turn into a French poodle instead of a wolf?!"
I buy that - and I'm still toying with a character with Visions. Even if being the SG and will probably never get to play him.
I just think the question is hard - I really like a lot of the aspects of Ars Magica. I always like the generel lab rules (including the experimentation), but what makes me tick is still the core rules of the magic system - especially with the 5th edition brush up and streamlining.
I believe that you're asking about the game in general rather than the changes to the newest edition in particular.
I think that players can wrenc h a tremndous amount of fun out of the spell creation rules. Specifically, if they understand the systems they'll be able to solve problems in a style consistant with their character's magic and add the flourish of their wizard's sigil on top.
The characters of my game come upon a guarded castle gate that they wish to pass through:
the Ignem magus would burn a hole through it
The Muto master would change into a bird and fly over it
the mentem master would command the guards to open it for him
the intellego master would use peering into the mortal mind to retreive enough information to convince the guards to open the gate for him.
Knowing the spell creation rules allows them to use their entire tool box.
I recently had cause to get to know the combat system extremely well, and I'm very impressed with it, especially when you start incorporating characters of different sizes. I found it simple to use and clever in its execution, and conflicts tend to work out as I would expect.
Also, the lab customization rules are very nice. I was hesitant to get my hands dirty and try them out, but when I did I was quite impressed. I haven't tried out much else from the Covenants book, but that discovery alone made me anxious to experiment with the rest.
err - if you read your gaming history you will find this is not so!
ArM1 (1988) has Virtues & Flaws.
Vampire the Masquerade from White Wolf was released in 1991.
White Wolf Magazine did cover articles about Ars Magica, and the early White Wolf company was formed c.1990
now, in 1986, Steve Jackson Games release GURPS - a point based system with Advantages & Disadvantages - but they too were not the first. Champions (c.1984) also has Advantages/Disadvantages
I can't remember other games of the time with such - I do remember Champions being startling and new in its design system
Just don't credit White Wolf with inventing the idea!
Man-to-man (the precursor to GURPS) most likely 1985?
PS. I didn't remember all of these dates, so I had to look them up. Man-to-Man was the first, combat only, version of GURPS. It is not mentioned on the GURPS wikipedia, which is just one more piece of evidence why I never trust wikipedia. Orcslayer, the companion adventure, was published in 1985. Alas, I've lost my copy of Man-to-Man over the years. And I didn't start ArM until 2ed.
I have played Champions since 1st Ed. , but i am more used to Ads & Disads as Power Modifiers.
And buying things like Hunted , which would be similar to Story Flaws i guess.
Having started with White Wolf , playing Vampire the Masquerade 1st Ed around 1991 or so ,
i did not know about Ars Magica until 3rd Ed.
(the Games Stores here being very slack on getting the product of smaller companies at times)
Not sure when i started playing GURPS , but i think it was with Horseclans ,
so after 1987 at any rate.
My group just fell in love with the experience per year thing. The aging roll is a nice thing too, when you get very old you don't get systematically wiser without loosing something. The system just feel right.
Beside of that, the way you get from grog to companion to magus can leave an awsome feeling when using storytelling technique. Some people just get attached to the grogs and the companion. In my firt saga (who's been going along for a little over 2 years in game time now) some session resolve a lot more aroung grogs and companions than the mage. Basically, the low-level magi just seem to always hurry to lock themselves up in their sanctum reading a book about some art. I really got to push them on adventure to get them out.
Fortunately, the story hooks let me get them since the characters pick them up!
I've just introduce the covenant's rule and when we use the covanant as an additionnal character it seem to really get my players attention. For now they really appreciate and were not even neat the initiation process!
Basically with the new system for damage... Instead of always getting a damagelevel / 5 you get a damagelevel per 5+Size. So if you are size +1 you get a light wound on 6 or more damage, once you exceed 2x6 = 12 you get a medium wound, and 3x6 = 18 for a heavy wound.
OtoH if your size is low... Size -1 would get for evert 5-1 = 4 ... So on 4 damage the small guy gets a light wound, a medium at 8+ and heavy wounds on 12+