time for me to gush about Ars Magica
The (default) setting is mythic europe in 1220. That is, you can take a history book a look up a lot of datials for setting etc, though it's not really necessairy.
Mythic means that the stories are true - there really are
faeries in the woods and demons of hell really are
out to corrupt you!
At the center of the game is the Order of Hermes (which you might know from oldMage - they imported it from Ars Magica!).
The Order of Hermes is the dominant (only?) order of magicians in europe. Ther powers are (potentially) vast, but to reach the pinnacle requires a lifetime of studies and wizards can live a long time indeed!
Ars Magica is centered around the magi, so every player (usually) has a wizard character. But the fun doesn't stop there. AM is designed for use with troupe style play, where most or all players have multiple characters.
Besides the Magus, most people will also play a companion - a non-magus, working with or for the magicians, often with a curious background and/or abilities of his or her own.
Also in the game are grog character, whch remind me alot of extras. Typically, these are guards/soldiers working for the magi (or for some companion!) and they are minor characters. This sounds boring - it's not! Grogs are bit characters, so you can ham it up with the roleplay and know that even if you fuck up, it's not you (main) character that did it. Likewise can you heroically charge the ferocious monster and be the hero of the day, dying for a greater cause - with a character that took 10 mins to write up.
Ars Magica even provides a reason for the characters working together and tolerating each other - libraries and laboratories, while necessary for expanding your wzard's magical powers - are hideously expensive, and so magi tend to live in covenants, which can become a sort of meta-character in itself. Working to improve your covenant really pays off in the long term, for by sacrificing time and effort now, you can build something greater than the sum of it's parts, and a strong covenant will have laboratories and a library that permits extensive growth for all of it's members.
mechanically it's simple, you add a few numbers together, roll a d10 and add in order to exceed a difficulty. This is alot like d20, because Jonathan Tweet wrote both systems, though I must admit that I find Ars Magica mechanically far superior. eg. it has no silly classes or levels.
Hermetic Magic is build around 15 Arts - 5 Techniques and 10 Forms.
The Form describes what you affect and the Technique what you do with it.
Eg: to cast a fireball (actually a Ball of Abysmal Flame), you use Creo (create) and Ignem (fire).
Eg: to turn a mouse into a horse, you use Muto (change) Animál (animal).
You can also combine multiple forms or techniques.
Eg: to turn yourself (a human) into a wolf, you use Muto (change), Corpuse (affects the human body) and Animál.
Magic can be used either as learned formulaic spells, or as spontaneously invented effects, the latter of which as less powerful, but since they are creted on the spot, much more flexible.
Some formulaic spells (eg Healing) are rtuals, meaning they take time to cast, and require somethng called Vis, which is basically concentrated magic.
Vis is also used for the creation of (most) magical items and a few other things. Ths is the catch-all for why wizards allways have such wierd items in their homes - many of them contain - or are
Ars Magica is also one of the few games I've seen where time passes even when you're not adventuring: Studies, laboratory activities and mundane practice is usually measured in seasons (of 3 months each) - this means that characters can grow old and start families or even die within the same storyline.
In a game I'm currently in, one of the magi is teaching his own daughter, who's likely to start her own family before she dies - and before he does, as wizards have a method of extending their lives far beyond what is normally alotted mortal men (though not to the level of celestial exalts).