It's the same driving force all MMOs have at later levels. Just because you are a warrior doesn't mean you may not want a skill from a different melee based class at some point. They just have to make the skills in newer classes interesting and you have the carrot. Just like leveling is never truly the driving force in games so far, classes won't be the driving force for everyone. I think a lot of people will just want to explore, kill, etc like they do in games now. It just has to be fun and SOE has someone hooked.Also, if the game is centered around just doing events/quests that gain you a new class... Whats the driving force for someone that is already playing the class they want to play and they have in their mind, the best cross passives and cross abilities for that class? The idea just seems to fall flat for a lot of people. Case in point - Final Fantasy. I have other classes leveled up, but I only play my Paladin. Its the class Ive worked the hardest to gear and its what I like to play.
The grind is there for all games. You just have to make it worth the effort. Doing it differently doesn't make it completely fresh and new but it's better than the same old crap.