For me, the most core elements of Ars Magica for me can't me limited to less than 4 [Edit: 5] categories:
- Troupe play, multiple characters, Magus/Companion/Grog types: I like it how the different character types are different and each have their niche. As long as you remember to have multiple players play grogs so they are not alone. Also remember that the magus' time is very valuable so that's why they delegate jobs to grogs and companions
- Lab work: One of the first things I remember about ArM that made it different, first time I saw is (it was when 2nd ed was new) was the existence of Magic Theory. I like it that not only can magi invent new spells to add to their repetoire, they can custom make them rather than use a limited list. Also devices, familiars etc.
- The magic system: Both the Verb+Noun as well as Spontaneous magic is IMHO a vital part of ArM
- Order of Hermes/Mythic Europe: To me the two elements both result in interaction, because I don't like isolated sagas. It's "The Order of Hermes" not "Individual Hermetic Wizards". I like it that the magi have colleagues, rivals, obligations, protection etc. It's "Mythic Europe" not "Mythic Fantasy World". I like it that the settign is based on real world historyk, culture, geography etc. which can be used for inspiration. And I expect sagas to interact with this and not isolate themselves.
- [Edit: Time: The passage of time, seasonal activities, and aging - especially the differences between mundane and magus aging. I like this, also together with the mundane interaction. Tha magus gets older and more powerful, the nobles change generations and forge alliances or enmities acreoss the lines and families. But the mioghtly dragon is virtually unchanged, so the magus can grow into defeating it.]