The most important thing is a reliable GM. The second most important thing is players who get along and communicate. The third most important thing is to focus more on plot and character interaction than on combat or exploration. Combat is quite frankly slow as hell in pbp and often requires strict observation of post order and that slows everything to a crawl. A focus on exploration tends to leave the players wandering around aimlessly and the game slowly grinding to a halt and dying as people don't know what to do and discussion of it is slow. More railroading than in real time games is probably warranted to avoid this problem, conversation with players and discussion of what to do in advance helps too.