Some things that are easy to do face to face are difficult to do in a play-by-post environment. One thing I've been struggling with is administering combat. In D&D, combat takes place in rounds, and everyone essentially takes turns doing something. There's only so much you can reasonable do in a round (which, technically, is supposed to encompass six seconds). In face to face gaming, it's easy to keep that straight. You roll up Initiative (i.e., turn order), mark it down somewhere and then let everyone do their stuff in order until the next round starts.
In a Pbp, that's more challenging. In theory, you do the same thing, but if you have to call out, "OK, John, now it's your turn," then your combats drag interminably. So, to keep things moving, it's better to have everyone call out their actions at once and then sort them out. Of course, this can, in turn, lead to a confusing situation where it's difficult to keep track of when stuff is happening, have players put forth actions for the current turn, etc.
So, it's a tradeoff. You get very slow, but organized play, or you get very chaotic play that is at least moving forward. In face to face gaming, it's not really an issue, because you get the advantages of both with the disadvantages of neither.
Le sigh. If you couldn't tell, I'm in the midst of a combat right now, and I got a little lazy about keeping it organized. Now we're all confused, though.