I do say this a lot, but when it comes to the OOC setup, make sure that you use a good layout with titles, lists and formatting. It'll help convey your message to your audience a lot better as it's more like documentation for your RP.

[@Darkmatter] did mention some good points with setting and plot, an aim, and the ability as a GM.

Your setting needs to be well thought out to the point where you can say definitively yes or no to most questions [i](eg, Will there be magic? Can I use this weapon? I want to go down this route? etc)[/i]. By knowing what it is that you want and sticking to that, you can prevent any complications occurring later on with bias, overpowered characters, and Mary Sues [i](Characters that fit TOO perfectly into the world)[/i].

It will also be 'your RP' so as a GM you need to know when to say no and when to say yes. Showing that you have a clear vision will draw people in as they will feel comfortable that the RP will work, and not become some hodgepodge of ideas that 'may' work.  

Finally, the Aim. Probably one of the most important factors that seems to fall flat on many people. As a GM you are creating the world, but giving a motive for the players in that world can be a bit difficult sometimes. I have the best example in one RP where for a few post my character didn't have anything to do as she was waiting for the GM to supply a new Aim after I completed my last. Instead of giving me a goal she wanted to supply exposition for the overarching story and thus my character had to wait for a few post for no particular reason. To be truthful, no one really knew what the goal was going to be as she only created the world and kept all the story cards close to her chest.

There is a fine line that you have to tread when it comes to GMing and that is between Roleplaying, and Story Telling. If you're too heavy into the story telling your players will notice it and feel like pawns rather than players. Therefore, if you mention in the OOC that there is a distinct objective of some sort, it will help portray your RP as something that has choice and opportunity. I won't go too much more into this as there is a heap more that I wouldn't cover for your needs, but by keeping this in mind while you create the RP, you will begin to understand where the parts are that you need to focus on.


Best of luck and let us know how you go.