An enormous focus in Diablo 3 has been placed on increasing the number of viable builds.
While each class may excel at some things that the other classes don't (Witch Doctor seems to be best at area control and damage over time, Wizard seems to be best at ranged AoE, etc.), no one class is supposed to be relegated to a specific role. No class has to be the healer, because each class is a fully functional independent unit, capable of sustaining itself and providing its own support.
That doesn't mean that you can't play as a pure support hero, though, if you really want to. Healing and defensive Monks look to be quite powerful based on their skill descriptions.
The distinction here is what the developers call 'viability'. Are there going to be emergent optimal builds that are just plain 'better' than other builds? Almost certainly. Does that mean that all the other builds are completely invalidated because of that? Absolutely not.
I am very bad at remembering where I hear a certain piece of information, but to paraphrase something said by either Bashiok or Jay Wilson: If the optimal builds are 5% better than the crazy wonky viable builds that people are using, that's perfectly fine. Some people love doing all the mathcraft to come up with that 5% advantage. It's only if there turns out to be some significant advantage that actually hinders the utility of other builds that they have a problem, and those problems will be addressed in balance patches if they emerge. The viability of creative builds is the main focus of the skill system by the devs.