However, while I might know my reasons for leaving a game behind, I'd like to know what other people's reasons. So:
1) Why have you left games?
I've left three D20 PBP games that I clearly remember.
In the first case, I was under the mistaken impression that the DM was following the rules for d20 Modern, but it turned out to be grim-and-gritty with a focus on realistic combat in heavy armor... using d20 Modern rules. I wanted to be a swashbuckler. Yeah... Didn't even bother to show up for the first session. I don't think I even finished making my character, though I'd been accepted into the group.
Another one, I made the mistake of being a science-type guy in a d20 Future game. Don't do that; the rules can never support such a character. Worse in sci-fi, whereas the rules might be better, the player's understanding of fields of science that
haven't even been discovered yet will, by necessity, be limited. We had this plotline where a ship had been shut down by unknown means, and I and a few other players kept throwing random quasi-scientific ideas ("it was a hack!" "it was a negative space wedgie!" "EMP!") because, not being 25th century (or whatever) scientists, we had no idea what was going on. The inability to connect to the plot caused players to gradually stop playing.
And in another one, a PBEM, we were members of a psychic academy, so we were all low-level psi-type characters. This was 3.0, but IIRC we were using sane, non-MAD rules. However, it took too long, some players (eg myself) were more interested in plot and action than in RPing feelings (as in, an NPC got mentally injured, and I was only interested in what did it... I wasn't going to have her cry on my shoulder because I couldn't care less) and the DM made too many NPCs (his own admission). Eventually it just sort of slumped apart. Too bad... the plot seemed interesting, and the sessions were actually fast-paced at first.
There's a few others I barely recall. I think one was StarCraft d20 Modern/Future.
2) Why do you join games?
Not being in a "meatspace" game at the time, or not being in a satisfying game.
3) What part of a pbp game do you enjoy most?
Just being able to play. Otherwise it's never as good as playing in "meatspace".
4) What part of any game do you hate?
Not being able to connect to the plot. It's too easy to have a plot where the players can't see what's going on. Mysteries are cool, but they should be easy too. I'd rather have to kill my way past a miniboss squad than have to spend another session talking to the bartender to solve the mystery.
Not having a plot! For whatever reason, several of the gamers in my group love running player-driven games. And sometimes they forget to clarify this beforehand. And if they do clarify, you have a bunch of PCs who have no connection to each other, not even a plot they can all follow, so you end up with one separate game per player, with each of us competing for DM face time.
5) What would keep you in a game long-term (ie: potentially years)?
Fast pacing. Clarify rules beforehand. Clear communication about concepts such as plot, player-driven or not, etc, before the game starts. (DMs must take the lead on this; players are sometimes shy, especially with a faceless e-person they know nothing about.)
And sometimes players don't really know what kind of game they're getting into, and should do more research before jumping in (then leaving and disappointing everyone else). Like that d20 "Modern" swashbuckler concept I got into. It was d20 Modern in the middle ages (I've run a successful campaign like that before; or at least the first few sessions were, and combat worked really well with minimal rules changes) and I had little idea of what "grim and gritty" meant to that particular GM.