If I ever made a MMO, I would have no character levels. I wouldn't have skill levels either. I also like the idea of players making their own skills, and maybe have the parts of the skills be like drops, maybe, not sure if I like that idea, but it is different. (Ryzom allows you to literally build your own skills, very cool.) Also I would have no classes... be what you want and everything you want.
To make up for the lack of grinding levels, be they character or skills, you would need various story lines. Add to that you would need some sort of arena and world PVP that made a difference, but that had limits on how many territories you could control, sort of like Necromunda campaign rules has. Obviously, there would need to be also a lot of titles, some that would be useful, and some for show. Guild Wars has a lot of titles.
Another concept from GW (and now DDO) would be henchmen that you could either recruit or hire and custom fit out in skills and equipment.
How to make it interesting in the really long haul... well you need more content and nice things to have. And there has to be some point to it all, which is to have fun. For some that is getting a title, others building a city, others crafting cool looking stuff, others being the top of the ladder in PVP, and others building alliances, and to some just hanging out in a cool space with their friends and maybe killing some baddies just because we all want to be a hero.