You've got the offense part down pretty good. What if they start focus-firing one of your guys? How do you heal? Are you going to have one specced resto? If not, your heals suck for mana and amount healed, and every time you stop to chain heal you're stopping DPS also. Survival, in spite of being relatively immune to CC for several seconds is where the problem comes in. Which is why most guys that do something similar run 4x shammies and a buddy playing a healer.