My 2 creds on the subject. I have run a number of play by posts over the years. I prefer moderated games in most cases.
Moderated freeform games allow the troupe to create a fairly stable story, with a stable of consistent characters, that can develop and expand over time... much like a novel or a table top game. You have a dedicated group of players who have made the commitment to post, have worked with you to hammer out a solid well developed background that is embedded in the campaign world, and in return you will provide them with a more developed story line than you can normally get in a free-form game.
Everyone looks at them and goes, "Oooohh That is really great, I want to join." The reasons the game is that great is because of all the specific time and effort the moderator and players have sunk into the game.
Let us face it. Most freeform players, show up, post a couple of times (or a few weeks), then leave. (That often includes the person who started the game....) That leaves you and the rest of the players in a lurch to write that person on. You many not be one of them, but many are. That is why we have a moderated section, so we can get a more solid play experience... one much more like the table top games most of us play.
If you want to join a moderated game, PM the moderator... begin the process. They may not have a slot to insert you into the story, (the moderator's story lines have already been worked out with the existing characters). But they might.