May I recommend this ancient thread covering this topic?
Salutor neatly sums up a lot of problems. I've experienced games struggling to get off the ground from everyone arguing about covenant design. I've also seen games suffer from people really not getting the original SG's vision and not buying in.
As there are always a few people desperate to play Ars, but games accepting players come and go infrequently, any new game faces a lottery as to whether it gets a big amount of interest from people who like the SG's idea or whether the only interest is from people who just want to play any kind of Ars.
House rules - it seems no two troupes play AM the same, and every spell is open to interpretation. I'm convinced that people must be very polite face-to-face as I never get arguments around the table as heated as people do online.
The two biggest pieces of advice I can come up with are prepare to lose a lot of players (happened in every PbP I've seen) and the more you detail the setup, the more people will know if the game is right for them or not.