Speed is an issue as well.
If the PbP game doesn't match the speed the players are expecting, it can be bogged down, players can loose enthusiasm, or players will drop. That applies to both sides, too fast and too slow.
Always announce potential disapearances or dead times. I let my players know when my posting will be slower, even if it's only a potential problem. Players are willing to wait on a GM if he tells them he'll be gone, but don't disapear too long...
Encourage your posters to explore aspects of their characters beyond what they might explore in a table top game. Sure, you don't need to RP every purchase, but while you're making purchases, perhaps 2 characters may have a conversation, while they are making out of character comments to plan something (which happens occasionaly).
Player Hooks are key. A good background with hooks can be used to get the player interested in the game. And you can often intertwine these hooks better in this environment.
A big key is to add extra touches, like some of the body language you can describe as people talk, or how you can split up the party and not slow down since you can still work with everyone at the same time.
All in all, there's a lot to it, and most games have some elements, and the best ones have all of them.