Well, this is an interesting topic. I'd like to consider myself very experienced with this type of thing. As said above, there are unique RPs, they just don't last. Not many RPs that I know of seem to last in general. People take up the big responsibility of making a RP without knowing what they have to do. It isn't easy. After joining a RP they like, people decide that they want to do things their way and make an awesome RP, but let me repeat and stress the most important thing here, it isn't easy. You have to prepare everything and a good plot is crucial. Even a good map helps.
I, myself, have my own fantasy RP that has been running for one year. I took four months total to prepare for it and I gathered a team of people to help devise ideas. Then, I advertised for the entire week, and people became interested in the plot and everything just went into place.
I found it very effective when I made people feel special. You know, before I go off topic here, I must say that according for a new RP idea to work, the plot and the people need to work together. That is a rare situation and so...great RPs are uncommon.