I'll start this off by saying that I am highly interested in running an Ars Magica campaign. I mean, of course I am; I wouldn't be putting up this planning thread otherwise, right? Right.
Anyhow, I signed up for the site, read through a lot of my books, had a bit of an argument with someone here...and then life got in the way. But the urge to run a game didn't go away, and I squirreled away some time here and there to keep reading and to keep thinking and to keep dreaming about magi. And now that life has calmed down and I've got the free time, I decided it was time to come back, ease myself back into the community here...and then say to heck with easing back in and just jump in with both feet and no life preserver. So I give to you all my attempt to run an ArM5 game, here, on the ArM boards.
I would ask that people please ask questions if you've got them. I will state up front that I don't know everything there is to know about this system, but I also cannot learn if I'm not being tasked with answering questions.
If I can get at least 3 people interested, I will get the request for the game board in. At that point I'll be able to put up the entirety of the character generation rules and covenant information so we can get started on the game.
And before I forget...thank you all, in advance, for at least taking the time to read this. 
Campaign Title: Et Potestatem Magnam...
System: Ars Magica 5th Edition
# of Players: Preferably 3-4, but will consider a 5th if the concept/characters are compelling enough.
Deadline: We can discuss this if/when we get enough interest and the forum is up and running. Magi will need to be finished first, with Companions and Grogs following slightly behind.
Starting Level: Magi will be 28 years old (Childhood to 5, Later Life to 10, Apprenticeship to 25, 3 years post-Gauntlet). Companions and Grogs will start at age 25 (Childhood to 5, Later Life to 25), but may advance to any age pre-game they wish. Keep in mind aging rules apply.
Advancement Rules: As per the canon rules. There will be some special stuff for pre-game advancement, but nothing too extraordinary.
Combat Rules: PbP Combat is tricky, to say the least. Combat in Ars Magica adds extra layers to that. I'd like to keep things simple and flowing for the sake of the story, and am open to suggestions on how to do that without players losing core mechanics or feeling like they aren't participating.
Dice-Rolling Rules: I'd expect players to roll their own dice for actions or spells, but there may be times where I'll roll things. This will be done only to keep the game flowing and not to disrupt the action.
Special Rules: Character creation will have some special stuff to it, most notably advancing post-Gauntlet for Magi. I want to allow as much as possible, but there's a firm NO on ROP Divine, Faerie, and Infernal. ArM5, Lion/Lily, HoH are all approved without need to ask. Anything beyond those three are ok, but I'd strongly ask that people give me a heads up so I can review things. If I'm prepared, the game is better for it.
Posting Rate: No less than once per week. The more you post, the more I respond.
Absences: All absences that are known about ahead of time (vacations, holidays, etc.) should just be posted with an expected return date, and your characters will be NPC'ed until you return. Emergencies notwithstanding, extended absences of 2+ weeks with no notice that you were going to be gone may result in your character not just being NPC'ed, but removed from the game until you return. These characters will lose AT LEAST 1 season's worth of time for not being around, and they do not get the benefit of performing activities in that season. It's harsh, but PbP is slow to begin with, and I want to keep things flowing.
Writing Expectations: Low to moderate. We're telling a story, so I expect people to be able to have some writing skill. I'm not expecting a novella or an in-depth dissertation with every post. But you should be able to be coherent and grammatically correct.
Text Formatting: I'm not going to be picky on formatting, other than speech should be bolded and internal thoughts italicized. I would like to ask, however, that the first line of any in-character post be the character's name; this helps me understand who is doing what.
Plot- or Character-Driven: This may not be a popular response, but I plan on starting with The Broken Covenant of Calebais. I do want to allow the players to drive their own story hooks/seeds as well.
Focus: That's going to depend upon what the characters/players are looking for. While I have a beginning story arc and activities that are related to that, I want the players to decide what they want to be involved in after that. Do they want to go politicking? Are they interested in wielding phenomenal cosmic power? Are they looking for a familiar? Taking on apprentices? After the opening story, it's up to the troupe as a whole to decide what they want to focus on.
Character Types: The best advice I can give here is to create the character you want to play. With 3 of the 4 ROP books disallowed, most of the concepts I don't want are already barred (Divine Magi, Infernalists, Faeries). I will state that all characters are in the Normandy Tribunal, and are members of their home covenant. Not visitors, not outsiders, not wandering troubadors looking for a good time. But as posted above, I should at least be given warning about stuff outside the stated allowed material so I can review first. I don't know everything, and I may come across stuff that might be a tad too complicated for me once I'm aware of it.
Campaign Description: A young Spring covenant in an idyllic mountain setting is shaking off the snow from winter, its inhabitants coming out of their labs and barracks. The Spring Equinox brings with it not only a feast to celebrate the coming year, but the seasonal Magi Council...and a few guests with business the Tribunal and other magi in the order want to see done.
Tribunal: Normandy. The Lion and the Lily is mentioned above, but I'm giving the exact tribunal so there is no confusion.
Covenant: We are going to use the sample canon Covenant of Semita Errabunda, with a few slight modifications. The Covenant will be located near the base Mont Pincon, which is about 20 miles south/southwest of Caen in the Normandy Tribunal.
Game Rating: I'm adding this one to the list of stuff to post about here. This is PbP, and we should try to keep it PG-13. Not necessarily E (for everyone), but close enough so that the casual observer and hard-core gamer can both enjoy what's happening.