One of, if not my absolute favorite, things about COH is that any team can be viable with the right strategy. Sure, there might be times where you wish you had someone to hold aggro, or maybe deal a little bit more damage, but if you play smart, you can succeed with any combination of 8.
If you had a group of nothing but healers in WOW, or any other MMO, you're not getting very far. Ditto tanks, or even DPS to a lesser extent.
However, and COH, a group of practically any 8 defenders is practically invincible. 8 half-competent defenders of any combination is almost game-breakingly easy to run. You could have a team of nothing but tanks, and be just fine. A group of 8 blasters can wipe out an entire herd, if you're careful with your targeting.