I write about my characters in the third person, mostly for reasons that have already been mentioned-- I'm playing a character, not talking about myself. I do tend to write in a style and voice indicative of the character's thoughts and attitude, though.
Aside from that psychic barrier, though, I find that reading post after post is much smoother in third person, because the perspective shifts from character to character more subtly. I never have to look over at the poster, and remember which character they're playing, periodically during play... instead, the character's name is usually mentioned at least once.
It's also easier to paint a picture for the other players of what's going on in the third person. For example, my characters usually have "tells" when they're lying or avoiding a topic or uncomfortable, things that they do unconsciously and wouldn't notice or mention if they were narrating. The character doesn't have the same perspective on herself as I do, and so she couldn't possibly describe herself as well as I will.