That's not to say that there've been no tough times. To be completely honest, there have been at least a dozen times in the lifetime of the Psionicle that, with a new update, I could've sworn that I killed the game. In OSM, there have been several periods when Jarval couldn't update for moderately long periods of time. Fortunately, both games have lived through those tough times.
I think the main reason for OSM's endurance is mainly down to a good and dedicated group of players. There have been times when I've not been able to update much (we're just coming to the end of one such period), but almost all of the players have stuck with it. It's also one of the best low-level adventures on the market, so some of the thanks has to go to Piratecat's writing skills. Speaking of PbP old-timers, I'm surprised that we've not seen more of Zhure in this thread. He's involved in three of the longest running games, so can definitely be counted as one of the "old-timers"
I think the Psionicle has benefited from all the PCs having a definite sense of purpose. We've got a vested interest in keeping psionics alive, and we've got a concrete way of achieving this goal. Plus, on a more personal level, Jansson (my character) has got something of a vendetta going with Jirlai. I'm not quitting until I've killed that bastard!
I think that many PbP DM's don't realize HOW SLOW PbP gaming is, and they bite off too much to chew in the initial set-up of the game.
Hell, I think a one-day adventure (IRL) takes over a year online, if the players get into it a bit.
Are there any veteran DM's that can add in their 2 cents, about how much you can hope to accomplish in a PbP adventure, given an average (or above-average) group of players?
Would you WANT to plan a memorable, far-reaching adventure, or would you want to keep it fairly small and focused on what the prime intent is?
There seem to be a few things that improve the chances of a PbP game having a decent life-span. All of the following should be read as though they'd got IMO after every sentence, but I think they hold fairly true across the board.
You need to keep things moving at a good pace, especially during combat, even if this means acting for absent PCs. It can get very frustrating having to wait for more than two or three days to find out what the results your character's actions are.
Generous XP handouts are a good thing. Maybe it's just me, but it's always good to have some sense of your character improving, which can be almost lost with the slow pace of PbP gaming. This is even more important in low-level games, where characters have fewer options available.
Don't do too much combat. It can take a long time to resolve a fight that lasts even as little as three or four rounds, so just don't have it happening too often (unless you're deliberately running a hack-and-slash game). Role-play intensive is the way to go with PbP, not least because everyone can take as long as they like to think up the right response for their character. It's a lot easier to do witty one-liners, as well.
Large groups slow things down. It's a lot easier to run a game for eight or ten PCs via PbP than it is IRL, but you've got far more chances for delays due to players not being able to post (holidays, family emergencies, etc). If you need to bulk up the group size, the Leadership feat provides an easy way to include vital classes in the party. Four to six PCs seems to work best IME.
Expect to loose a player or two. Be ready to recruit a new player at short notice, or have some alternates lined up from the start. A couple of PC suitable NPCs can really speed up this addition.
Have a goal for the PCs. It doesn't have to be the end of the campaign, or anything all that major, but the PCs should be pretty certain about what they're trying to do. Indecision can slow things down a hell of a lot, or lead to accusations of favouritism or railroading if the DM picks up on one character's suggestion.
I've noticed that here on ENWorld most PbPs should probably stick to shorter adventures...though on my own board, where its simply myself and a fairly large group of friends, our PbPs move very quickly. I'm running a large amount of games there, and we've been doing it for about a year and a half now.
I have to say...coming to ENWorld and seeing how slow the PbP games went here surprised me a lot.
I've got to agree that things do seem to run a little slower on EN World. My Buffy game (hosted on it's own board) has generated a lot more activity in the last three months that some of my EN World games have in double that time.