(edit- Pew! Quite a long one, sorry about that. Guess you struck a nerve!)
I agree with a lot of the above, but not in all points, and I think I've played in some of the games mentioned, or ones just like them.
The first and foremost is the posting format itself, the style of the board. Play by E-Mail just doesn't work, imo, because it's too awkward to follow. Nor do one-thread formats where everything crammed into a single continuous stream, or formats where it's awkward to navigate between threads. A PbP board like this, IF the ST can create new threads for different storylines/whatever, is ideal.
(Some RP boards allow multiple characters for single players, with multiple avatars/portraits- now you're talkin'.)
This all feeds into the "post early, post often" theme mentioned above, where the easier it is to read, the easier it is to respond.
The idea of a single NPC (or PC?) to "lead" the group isn't a bad one- it does avoid many IC conflicts, and the predictable "start up" pains that come with some Spring Covenants- charters, what is a group "necessity" vs individual concerns, etc etc. Some like the give and take, others just wanna blow stuff up real good. (And you can always kill them dramatically later.)
Combat- well.. before I go there, let's talk about advertising. There are many, many styles of play. Imo, THE MOST IMPORTANT THING for any game, PbP or otherwise, is that everyone has the same sort of gaming style, that they are expecting the same thing from the game as a whole. Some are powergamers, turning themselves into dragons (ahem), some are labrats, blowing into their own thumbs and getting bigger, and Bigger, and BIGGER, but never interacting, and some are all about The Human Drama... and those are all good, for those who like that. The point is, when you advertise the game, be clear about what you expect, and what the players should expect.
In that advertisement, also make it clear that AM does not tend to move as fast as, say D&D. Also emphasize that character death may be a real possibility (if so), and that any one character is not as large as the story itself- that's a big part of AM to many players. Too often, people come in expecting something different, either from other games or just from how their troupe plays, and when they don't see it, they leave. (More on this below).
Posting length- I'll disagree with YR7, some people are authors. The key, as above, is to find a middle ground that all can live with. And what you can live with. Decide what will make you happy, and post that in the advert. Chase away those that will drive you crazy, and be happier for it.
Now- combat. When I advertise, I make it clear- IF combat starts, there WILL be one GM post/day to advance that combat. I sit down about the same time every day, and if a character has not posted, they WILL be NPC'd "reasonably"- not disappear for that turn, nor charge headlong to their dooms, but an "attack the nearest enemy" sort of action, unless their character has shown themselves, through RP, to have a different habit (or the player left the ST a note!). If a mage, then maybe the same spell they cast last turn, or the last one that seemed to work. If possible, perhaps a vague, "invisible effect" spell that can be filled in if the Player returns before tomorrow! Whatever.
(See also PbP vs Tabletop, below)
The reason for this, explained in advance in the original advertisement, is three-fold: 1) It's not fair to everyone else; 2) It's far more engaging to read regularly, and easier to keep track of what happened "30 seconds ago", and 3) We get back to the lab with the vis we got that much sooner.
Characters- When a person joins a game, any RP game of any sort, unless they truly know the ST, they create a character that they imagine will be fun. They have a scene, or a story in their head, of that character doing "great stuff" appropriate to that character. IF the ST has a particular theme or storyline in mind, then share that in advance! Nothing worse than a huge Flambeau in a purely political Saga, the flacid lab-rat trying to fight the Mongol Horde, or an evil-tainted, strong-gifted, fae-faced, notorious hedge-mage ex-misc with the Evil Eye in a saga revolving around public relations. "Challenging" is one adventure- an entire saga of that is just torture. Don't have to give spoilers, but give the players a hint where you're going, so they aren't left behind.
The flip side is that the ST should try to incorporate Characters' personal elements/goals into the Saga. If the ST has to create a different storyline for each mage, that's 2xX storylines, one for the mage, one for the Companion, plus any plotlines they had in mind themselves. Otoh, if the ST can (subtly?) invite magi/companions toward a more limitted number of expected themes, then that reduces the number of storylines needed to give every character their day in the sun.
Likewise, In Character, both magi and companions should grow organically out of the covenant and its needs and preferences, not be shoe-horned into the storyline simply because they flash a Player Character's Card. Screen out potential nightmare characters if they don't fit the storyline/premise. Too often, characters (magi included) would have no place being accepted into an existing covenant - but there they are, the proverbial turd in the punchbowl. Let the players know - any character must be accepted by the existing characters - (this helps with limiting storylines too). Better, by IC invitation only (ie, the covenant goes looking for specialists to fit a certain need, etc.) You want to run a lay priest? Find a covenant that wants him, it's that easy.
Number of players/characters- I find that in PbP you want to have one or two more than you want to run- RL or flakiness will thin the herd naturally. Me, I think that anything less than 4 limits the interaction, so I go for 5-6 players in the game. Making shield grogs for other magi, and encouraging a [variation between the function of the companion and mage, is also a good plan to be more easily included in storylines. (A berserker companion and a Flambeau mage, or a healer and a lab-rat Corpus mage are not so good combinations.)
Be prepared to run micro-adventures to keep players amused. A single benign but annoying faerie in the lab/kitchen, or a travelling merchant with some rare items from the exotic Levant - something, anything.
PbP vs Tabletop. YES! Lose any OOC references to the rules, game mechanics, or side comments in the IC threads, as much as possible. You are not seeking a transcript of a tabletop game, you are writing. Maybe it's bad writing, but it's writing- let it appear to be that, and it'll read, and flow, and feel, that much better.
Characters post intentions, not success/failure. They "try" something, but the ST will let them know how that works out. The players don't need to know the modifiers. If the player complains that they then have nothing to post about, either lose them (my choice), or tell them to expand their IC reaction to the success/failure of last round.
Don't worry about "posting in initiative order"- that's a tabletop convention for the convenience of the ST. Each player/character posts as they can, and the ST makes sense of it in one comprehensive "OK, this is what happens!" reply, followed by the bad-guys' actions, then all the players post again. (Waiting for "initiative" is an OOC tactic and GM tabletop convenience, nothing more.)
The ST rolls "all" the dice. Another tabletop time-saving convention to speed things up. In PbP, the GM sits down, makes all the rolls, makes sense of everything, writes it all up, done deal, next!
Description- try to describe the situation fully. Put yourself in the players' minds, and think about what you would want to know- open flames? big/little trees? Is the floor dirt, wood or stone? etc etc- set up the image clearly and distinctly, and the world will be a happier- and more lifelike - place.
Pro-active. This is a tough one for some, those who are used to the "Question & Response" of tabletop. There is no time for a "What do you do?" "I open the door" speed of play. It may take some time, but foster players to post a "plan of action"- if they plan beyond an intervening incident, no great harm done. But meanwhile, the plot progresses (and the ST might get a hint at some player fears/concerns that they can weave in!)
Arguements. If two characters are arguing, make sure the players are talking at the same time, letting each other know it's IC, and nothing personal.
If it's a rules dispute, step in quickly and make an executive decision. Don't let one player claim they "know" the rules, how one part of the Code must be interpretted, or how the Enigma works, or what magnitude a certain spell effect must be. Lose them if they persist in their delusions of infallibility.
NEVER think that a troublemaker is worth saving. They will bring the entire game down around them. Love the game, not the player. Hard love, lay down the law, and lose them if they don't get it. There are one or two more people on the internet who might be willing to play, trust me. (Besides, after the player is invited to leave, killing their character in dramatic fashion is a great plot device- nothing like a good enemy to keep the interest up!)
While tabletop is all about action with story tying it together, PbP can be the opposite, which fits AM perfectly. While "hogging the spotlite" in tabletop can be annoying, an internal monologue in PbP can be a great scene builder. While tabletop is all about rules, PbP can put those all "behind the curtain", and seem to be nothing constrained than a freeform writing excercise.
When the players forget the rolls, the roles are all that's left.