Committed players, a committed DM, and no characters based around a "look at meeeeee!" concept. All the players should have an equal part in the adventure, not be one player's sidekicks. Likewise, disruptive characters (or even worse, players) are a big issue for me.
I don't love a million house rules, but so long as they're pretty easy to understand, it's not a super-big deal. The closer the game sticks to the rules in the SRD, so I can look things up (and participate) from work, the better.
Beyond that, it really depends on the individual adventure and sometimes the setting.
But the most important thing: A fast rate of posting. I know that stuff happens, but if you look at how many pbp games begin, compared to how few make it even to page 10 of a thread, it's really depressing. The less often people post, the more likely (it seems) that the adventure will just fizzle out.
When I run my Ptolus game later this summer, I may try a hard requirement of two posts on every weekday and one post per weekend day and screen accordingly. I'm not sure if it'll work, but it can't do worse than so many others here do.