Syn's 2 cents:
I count myself included in what I'm about to say, hence what I say is in the third person - not first.
1) There's like, nine active RPG posters who are involved in rpgs at this time. Out of these nine, five to seven are in more than one game. That's less than nine people spread across 6-7 RPGs. Most of these nine people tend to play the same characters, therefore, they all tend to interact with one another with the same characters. Despite everyone's talent, it tends to get old.
2) In every RPG, it tends to be the same people running the 'major arcs' that incorporate the whole RPG and essentially classify that season of a specific RPG. Because of this, the major threat that opposes everyone in the RPG also tends to only be beaten by the major heroes (because that's the way it is in the comics, generally), so the people playing the big league characters become pivitol, while everyone else just sits idle until the arc's climax ends.
3) It's not the availability of the character's that drives interest in an RPG - it's the availability of the posters who control the big characters in an RPG. Or, not even the big characters - just the characters who are pivitol in a specific season of an RPG. The more available the posters playing the core characters are, the more interest in the RPG there will be.
4) The major arcs driving a season tend to be centric around a certain class or group of characters. It may start out as expansive, and open to any character, but it always ends with a specific group of characters as important to the arc at hand.
5) A lot of the recent major arcs tend to be action and disaster - big monumental threats solved by brute force and perserverence. Not everyone has a character who has super strength, speed, invulnerability, etc. What happened to threats that occur on a psychologic level - threatening the characters' status or roles, as oppossed to their physical stature? It always seems that at the end of these arcs, the only ramnifications are characters die (who will be revived next season anyway), or easily restored alterations to the status quo. It feels like the arcs serve no purpose to the rpg as a whole - the arc just becomes a way for characters to have interaction on a major scale together, which is fine, but after multiple seasons in multiple rpgs? It gets old.
5) Not every major arc needs to follow what's happening in the comics right now. We just read it - I don't want to re-enact it with slight varients in the RPGs. (This is not just referring to DC RPG - it's been happening since I've been here).
These are just some things that come to my mind off hand. There's probably more, but I think that's enough from me right now anyway. This is not meant to offend anyone, nor is it directed at any poster specifically. I'm guilty of these just as much as anyone else, but it doesn't mean it doesn't affect the RPGs. Hopefully this helps.