I never got to see any kind of Mage chronicle that involved detailed rules about large amounts of Tass. When I was a Storyteller, I ran some games where Mage NPCs had large amounts of Tass, but I never managed to get the players involved with Tass management.
By contrast, in Ars Magica, there are fifteen kinds of raw Vis. (Mage Revised has Resonance for Quintessence, but Ars Magica had it years earlier.)
Ars Magica sometimes requires more than two scores, with one as a requisite, but so far as I can tell, Ars Magica has no way to allow casting two different spells, with different verbs and different nouns, at the same time. Mage, by contrast, allows various sorts of “conjunctions”; a Mage effect can have a Prime 2 effect fueling a Matter 3 effect while a Correspondence 2 effect channels information to a Mind 1 effect, and the whole thing is Matter 3 Prime 2 Corr 2 Mind 1.
A great deal of the complexity of Mage effects would become available in Ars Magica if Ars Magica spells could use more than one verb and more than one noun in the same spell. Suppose you have skill 10 in Verb 1 and skill 15 in Verb 2 and skill 5 in Noun 1 and skill 10 in Noun 2. Your basic Verb 1 + Noun 1 total is 15; your basic Verb 2 + Noun 2 total is 25. If the wizard just has to add all his totals together, his skills get higher as his magic incorporates more elements; 15+25=40. It would be just as easy to cast a level 40 spell with two verbs and two nouns as it would be to cast either single spell.
Another approach would be to condense Creo, Perdo, Muto, Intellego, and Rego into a single skill – call it Ago for “I do.” In that system, one would need to have a total of Ago and Noun adequate to the requirements of the spell. This would weaken niche protection, of course.
For historical reasons, TRPGs tend to feature extensive niche protection. Most TRPGs are designed with the idea that four players should sit down with the GM and each player should pick a class that is good at one specialty but bad at the others. Thus in D&D you get Fighter, Cleric, Magic-User, Thief. In Mage, you get nine spheres, with enough weird exploits to ensure that a group of four players can select different spheres and use different gimmicks in play. In Ars Magica, you could easily design a group of five specialists, one for each of the verbs.
However, if there is just one magic skill (not unlike GURPS Magery) then the game balance of Ars Magica is thrown way off. For example, Ars Magica normally uses the notion of spell penetration, which is very annoying when a starting Creo Ignem specialist tries to throw a Level 25 fireball at (e.g.) a faerie with a faerie might of 20. The roll might be good enough to cast the spell without fatigue, but not good enough to get past the faerie’s might. On the other hand, the specialist might either have a specialized spell penetration skill, or else the insight to cast a Level 5 Creo Ignem spell. The faerie would probably be vanquished by the level 5 spell, since the casting total would provide spell penetration. It’s not a terribly elegant system – but then again, I can’t think of any detailed TRPG that really deserves to be called elegant.