Roleplays always end up in one of three extremes. Either everyone is a god, everyone is a half-demon, or everyone is a bimbo. The odd occasion, all of the above. Regardless of setting, plot, or anything, this will always be true, simply because people who WANT to roleplay WANT to be these things. And the story proceeds to go nowhere because they think petty fights and petty romances are cool. Or that being a loner half-wolf with four arms, two keyblades and angel wings is in itself cool. (Spoilers: You're a dipshit if you think so)
Of course, regardless of whether or not this forum or another refuses because of these pretty good reasons, most who want to roleplay will simply refer to these reasons as good reasons, because they think they know best. So ultimately they'll just bugger off elsewhere for their fix anyway, and fall into a group of others who think the same way, and use that to fuel their reasoning that it's a good and/or intelligent idea.
The closest I've come to witnessing or participating in a 'good' forum RP was more or less a gimped DnD game. Organizer would roll a d6 for player actions to determine if they succeed, fuck up, or royally fuck up to the point of killing themselves immediately. Of course this kind of RP is self-aware that it isn't meant to be taken seriously, while a RP forum itself fails to be.