Free-for-all posting, in my experience, runs into major issues as soon as someone can't continuously monitor their e-mail all day. Whoever can't post all the time gets kind of shunted off to the side, having little effect on the flow of conversation. It is usually disheartening to have that happen to you.
So, I usually see posting quotas ("No more than X posts per day"), time limits ("No more than one post every X hours"), or an outright post order ("You cannot post until the person before you posts").
In general, the GM cannot keep up with a large numbers of players at free-for-all speeds for resolving rules-based actions. I often see play split into a couple of threads - one for talk that the GM doesn't have to pay up-to-the-minute attention to, and one for actual in-game actions that require dice rolls and GM adjudication. The actions threads are the ones that most need a post order, as it is analogous to initiative order, and everyone ought to get a turn.
You may also need some agreement on narrative control, and that depends on player trust. If everyone can only speak for themselves, you tend to get a whole lot of short staccato posts. If the players allow each other a bit of narrative control, you can get longer posts that are more like storytelling.
