Roleplays require planning, possibly more so than a fan fiction due to the dynamic element of real people playing the main characters. The planning must address the plot, where we want it to go. Too many have died because they didn't have a well thought out plot, and this disappoints me greatly. You must have a good idea of location, a familiar and preferably in-game place to at least start in. You must also have something be happening, an event to involve all of your fellow RPers. It must be original, exciting, and leave opportunity for input from your companions.
Often the group or groups of RPers form the protagonists, and they must combat the antagonists in the same format as novels. They can be heroes, anti-heroes, or just ordinary citizens, as can the opposition. The main conflict should be between the two groups, with internal struggle to add realism. The characters must engage in dialogue that both exposes their personality and advances the plot, but not too quickly. They must make decisions as to how to solve the conflict occurring at the time, and do so in a way that is original.
The characters, as I said, can be good or evil. Either are interesting and fun to read and write, but the nature of the characters will change dramatically with the choice between good, evil, or in between. Often the groups will have both good and evil characters, adding internal strife and complicating the conflict, and this makes things all the more interesting.
As I said, the plot can't be freeform. You can't just tell your RPers to go do something, you must present them with a situation in which they can interact and resolve the original issue. This is a key mistake many RPs make.
Many RPs are set in a post-oblivion crisis world, but this is not always a requirement. It leaves room for more creative plot and undecided outcome, but pre or during the crisis RPs have equal opportunity for success. Not to mention RPs centering on the Morrowind plot, or ones that have nothing to do with the main plot of Oblivion.
Well, I have written too much already ^^. Please input your thoughts on what an RP needs to be successful. I only touched on plot here, but feel free to address characters, prose, rules of thumb to be remembered, etc. Tell us anything you think we should consider when making an Elder Scrolls Roleplay because I for the life of me cannot seem to organize it all in my head.
Please do not restate things mentioned in the guide to RPing by Illusionary Nothing, this is more for RP makers than RP players. I intend to summarize my ideas and yours and edit them into this post so that people can look at the information without all the hassle. Thank you.
Oh, and any constructive criticism on why this should never have been made are welcome. I hope to address them satisfactorily. Thanks again.
EDITS: @FC4 I agree completely. This is not a cut and dry, guranteed success kind of thing. You will have better chances with a plot, but it's just that, a chance. As you said, the past has shown many good RPs that have been free form, but a few don't make up for the hundreds of dead ones.
Also, players should give input and influence the plot. Whether by pm, IM conversation, or subtle in or out of character hints. The creator cannot hold a dictatorship on the story, the players have just as much say in what goes on.