I've played by post a few times before, always as a GM. The best games I've experienced had very involved players, that studied the rules, were very patient and it was obvious they were really into the game; their posts were always on the mark, both thematically and mechanically speaking, always on character and with full knowledge of the rules. The worst games I played had players that did not keep track of their progress, not in their character sheets or maps. They did not bother to read or understand the rules, even when they were given resources and access to the core books. They were not constant or thorough in their posts, always posting every four days or more, with no awareness of the current situation and posting entries in a very incomplete way as in "I attack", yes, but who, with what, how? I think roll 20 is a great place to play by post, and if there is any doubt about position, situation, etc., the players just need to check the map and log. The great advantage of playing by post is that you don't need to keep a tight schedule of when you play, just set a number of posts per week and stick to it. Usually, the best play by post players I ran into were ladies and gents in their mid 40s; the worst were younger players that thought play by post was some kind of video game (and I love video games, mind you).