I have never GM'ed, IRL or PBP, but these are some of my observations.
One important thing about PBP is to have a really good character background, and a clear expectation of what your character will do given any set of circumstances. Not only does it make it better RP for you, but if for some reason you can't get online and it does happen to be the middle of a combat, the GM can take over easily for you. I've had a couple of campaigns where I was unexpectedly kept away from EN World, and upon returning, found the GM had acted for me. My first reaction was, "Hey, wait a minute!" but then I read what had happened and I said, "I would have done the exact same thing." It also makes it easier for RP (combat and not) if you are expecting to be gone--the GM or another player can take over easily. This makes the game flow much more nicely for everyone involved.
One thing that I don't like in PBP is the tendancy to hold on to the PC of a player who is no longer in the game. I admit that I'm the first person to cry foul if something is done that doesn't make sense in-game (or in-character), but it just becomes a drag to lug around "extra" characters. Not only does someone have to take over for the missing player (often this falls to the GM), but eventually you get to the point where the missing PC only ever shows up to perform a particular duty, or set of duties. For example, the fighter only appears in combat posts, or the cleric in post-combat when someone needs to be healed. And that is just about as intrusive as you can get, in my eyes.