Having participated in a number of roleplays both outside and inside NS (and by outside, I'm referring mainly to Star Trek Simms where the entire focus of the community is on the RP), I can tell you the one thing that is difficult about any RP is keeping people interested.
There are a number of scenarios. For example, the GM could lose interest in maintaining the RP (which has happened to myself once or twice). Someone could have RL (or other NS duties) interfere, which means they can't participate, and in some RPs that means that it's difficult for them to get themselves back into the story. Or perhaps the story itself could be running a little loose, either it's too complicated, too dull, or just going too slowly. Some or all of these factors mean that specialised RPs find it difficult to keep going.
I do like the idea of having a main easily accessible RP where people can jump in or out of the RP easily. You could also introduce RP's that lack a main story and instead are set in a well defined area, where the main story comes from the interactions between characters, and those interactions form the story. In both these areas, the GM becomes less involved and is just one of the characters, RL/NS interference means you just bounce out and can reappear later without too much difficulty, and the story is what you make of it.
Anyway, that's my hastily written two cents
Well, you've gotten a lot with those two cents. I'm gonna have to call you an economic genius.
Jokes aside, the New Hyperion main RP has its own section of the forum with a few subforums dedicated soley to specific IC and OOC threads like factbooks, story threads, and OOC discussion. We have a tendency to create an entire nation to use but also have main characters to use like politicians, generals, etc., the big important figures (E.g. the Aesir Empire has a Jarl, three Kommanders, and nine Chieftains, thirteen important figures at the least. In PvC, the aforementioned RP, I've also thrown in a lead weapons engineer, Akaz Sveigdir.). And going off of what Wuufu (awesome avatar by the way) said, we don't use a defined story. We make the story by interacting with one another. With there being such a loose story, we do have the potential to jump in at any point, but it also can be that you get so involved in what's going on that you can't just jump out. This is why we have a special group on our forum called the Roleplayers Guild that one has to apply to enter into if they want to RP on our forum. In the application, we make it known that activity and participation are requirements for reoleplaying and if these requirements cannot be met, don't even bother applying.
As a promotion to our RPs, we kinda make them the star of the show in the region. Sure, there's the Body Republic, WA Comm, Inquisition, Media, etc, but none of those are on par with the RPs. When we have the main RP going, we make sure
everyone knows about it and that they know they are free and welcome to join at anytime. We do have rules set in place to make it fair for everyone but still enjoyable.
If you need further explanation, please don't hesitate to ask. I'm kinda writing this last part in a rush, so I may have left out some key things.