Well part of that is the fact that the beta is locked into normal difficulty. Once the harder difficulties open up, it will get much more difficulty all around. I've maxed out all chars in the beta (and gotten all beta achivements), and after a certain point, its all way too easy for many classes. Some classes are still difficult, but I think thats because their better abilities unlock on levels above the cap in the beta (at least they'd better, else some classes are going to suck).