Because I don't care about society's rules, I'm giving you the "Dos" of making a Forum Adventure.
Make us care about characters! Having tons and tons of characters may seem cool, but if they're boring, flat, characters, the story tends to suck. Having a more limited cast of characters makes it much easier to develop each one and give them a distinct personality. Of course, you can have a lot of characters, but the more important characters should have strong personalities and overshadow the less important ones.
Also, watch your grammar. A more serious point of a story can easily become stupid just because the writer was too lazy to use the right form of "there" or didn't capitalize properly. Sure, most people won't really care, but it certainly helps! I like to read what I wrote out loud a few times to make sure none of my sentences are super awkward.
Make a good template for the characters so that people can provide you with as much information about their OCs. You especially want to ensure that people can give back stories for their characters, because if you don't, it'll be quite tough to write them in the story. Trust me.
Good luck on your adventure! ^u^