It is pretty easy to level up one class to 50...in a game that actively encourages you level up multiple classes. You may think you're done when you get to 50, but the reality is you're just getting started. If you want a grindier(why god, why) curve, you'll have to reduce the burden on the player by limiting what can be ported over from other classes.

Actually that doesn't sound that bad. Getting one class to 50 takes longer but you could only use the skills of other classes of up to say...level 20? Sooner or later they'll have to address the inherent balance issues of cross-class abilities, and that sounds like as good as any way of doing it.

That's all assuming that there's more to do than aoe mobs in a group mindlessly, of course.