Wizards recently posted an interesting Gamer-Zero video that does a good job of defining monster roles:
I may not be thrilled with all the implementation decisions made in the past few months, but 4e does look interesting. I very much like the idea of multiple variations of zombies and such. There can't be too many toys in the writer's toy box, in my humble opinion. Variety is the spice of life, they say. Time will tell.