I have a few ideas:
1: Pace. Every time I see people advertising PbP games with weekly turns, I don't hold out much hope for them continuing. To me, PbP is a daily posting game, or posting at least 3-4 times a week.
2: Too many games: In general, especially on forums dedicated to PbP gaming, many players are in too many games. I know why they do it (#1 above), but it makes it hard to give the game the attention it needs.
3: Every voice counts: Too often DMs wait for everyone to chime in. Well, especially for PbP, if you do not post regularly, your voice does NOT count.
4: Lack of detailed posts: Especially in PbP, or any online medium, HOW you say it / DO it is at least as important as what you do. The added depth of a post helps make the characters seem alive.
5: Pace: Saying it again as this is, to me, the most important point. Get the pace high and keep it there.