As I briefly noted below, I've just gotten involved with a play by post AD&D2e Planescape game. You can read about it here. With only a little more than a day under our belt, I think it's shaping up nicely. The other players are enthusiastic, and the DM (noisms from the monsters and manuals blog) is setting a wonderfully old school tone.
I'm really trying to play a type of character that I'm not sure I've ever played in an rpg before - someone who's very precise and bound by harsh cultural customs (you know, like duty, honor, and hierarchy). In other words, someone who's lawful neutral. It's fun but definitely challenging. Especially because I don't want to piss off the other players who I don't even know with an overabundance of inaccessibility and arrogance. (It's all in character - I swear it's not me!)
It's hard to say whether this type of game has the staying power of in person games (though who am I kidding about the staying power of these). But I'm definitely enjoying the writing aspect. I'm trying to look at my character as I've heard great comic book writers talk about their characters: It's all about distilling the character down to the handful of core elements that make that character cool and make that character tick. So, I made a list of some elements, and I'm going to try to keep them on the screen next to my browser as I post.
At the very least, the writing saves me from embarrassing voice acting.