I also liked the direction they took by stating up front that Mages are the star characters and that don't expect to see any of the other character types to be in the limelight when a Mage is around. The use of latin for spell names and the insistence by the book to use latin names when using spells is a very nice touch. Ars Magica is different from your standard RPG in character creation. Mages are individuals in Mythic Europe that can cast magic. Companions are the nobility of the social order and the ones who usually act as a go-between with the Mages and the rest of the mundane world.
Grogs are the scut workers.
The heavy lifters and the strong arms of any covenant. Tasks are rated in 3 categories. GM determines the difficulty of the task with higher numbers indicating a more difficult task. Since Mages are the most important people in Ars Magica, the book mostly details their creation and the use of Magic. Frankly, this rocks.