I think a lot of DMs hangstring themselves as well by choosing some pretty "out there" scenarios and games... and it turns out the fun for the players only extends as far as character creation. I honestly think most players in PbP don't really like the actual roleplaying aspects of the game, but prefer to just create character concepts.
So when a DM says he is creating a game for "12th level characters using only clerics in a Birthright Campaign and the rules system will use an adaptation of Arcana Evolved with only these 16 house rules, and the module is long enough that it'll take characters up to 18th level when it is done"... you'll get a bunch of people coming forward just for the challenge of creating a character for that particular game... but none of them really want to PLAY that game once it starts. I always find it amusing when players write in their character descriptions for a new 1st level game the Prestige Classes they are aiming for... AS IF the game would ever actually last long enough to gain enough levels to start worrying about Prestige Classes. But that's what draws the players in to sign up for these games... creating the character - past, present and future.
Another problem of course, and one that will destroy a game nice and early, is the insistance of playing PbP the same exact way you would play around the table. Which really, really suchs, because the d20 rules in PbP are a PAIN IN THE BUTT!!! Many GMs get so caught up in the rules of the game that they take all the joy out of playing it by requiring every roll to be documented using Invisible Castle, every combat has to be officially drawn out using proper grids, every ability has to be used right down the middle using only official rules etc. etc. But the problem is... getting that wound up in "playing by the rules" when in a PbP game just slooooooowwwwwsss tttthhhhiiiiiiinnnnnngggssss dooooooooooowwwwwnnnnn.
For my money... these games are invariably so short and player turnover is so high that you should pretty much DM the games by the seat of your pants and without barely ever looking at the player's character sheets. Just wing it. Would it be more interesting if your players spotted the orc hiding in the trees? Yes? If so, then ignore making the characters roll Spot checks and posting the Invisible Castle rolls to the thread... just let them spot the orc! And see what happens from there.
I know that in the game I'm currently DMing... there have been numerous times where I've fudged the rules just because it helped push the game along. Since an encounter that would take 10 minutes around a table could take a full month on PbP, I'll ignore plenty of rolls just to keep the game moving. Did the players fail a Search check? Well how much faster can the game continue if they had passed it? A lot faster? Then gosh darn it, I'm saying that they passed the Search check, regardless of what my computer die roller told me! Otherwise the party is stuck in this one spot for an extra week and a half of real-time as they go looking for another clue. And as far as the players are concerned, THEY PASSED the check! They don't know! And they don't care! They want the game to move forward just as fast as you do, because being stuck in one place for over two weeks of real-time because the DM won't progress his game since the players haven't figured out his "intricately designed plot" yet... is just asking for the players to get bored and stop posting.
PbP is a prime example of K.I.S.S. - Keep It Simple, Stupid!
DON'T start a module that is so long that there's no way to ever finish it. DON'T require players to learn 75 of your house rules because you don't like d20 combat. DON'T put players in a location where it's impossible to restock the party should some players quit. DON'T be a rules nazi, because most PbP players just care about writing interesting stuff - not rolling a bunch of computerized dice. DON'T start a game with less than six players, because invariably at least two will quit within two weeks and you'll end up at the four players you originally wanted to DM for in the first place.
And most of all... DON'T start up a game as a GM because you're actually a frustrated player... because DMing IS NOT A REPLACEMENT FOR PLAYING! They are NOT the same! You will not lose your jones for playing if you GM instead. Only GM if you have a nice quick story to tell and like the idea of writing narrative three times a week. Then, AND ONLY THEN, should you volunteer to GM. Otherwise the game will die within 3 months just like Whizzbang Dustyboots put forth.
There was one game that got to the final combat and the GM didn't have time, so it was effectively over, but I didn't count that one..)
