in TBC healing was more "powerful" than DPS, in that you could outheal every classes DPS up to a certain point (running oom). This is why druids were the kings of arenas in TBC, since their mana regen and HOTs were so powerful that you could never drive them OOM. Its also why top arena matches sometimes took 30-90 minutes.

Blizzard didn't want this in WOTLK so they made dps "more powerful". You can no longer outlast DPS by healing, you will always die first. Healing is more a survival tool to last long enough for your side to take out someone before they take out someone.

5 priests will lose every single game because there are too many tools to "shut you down". DKs with grip/silence/amz, ret pallies with stuns and bubbles, mages with CS, warrios with stuns, etc will almost always kill one of your guys before you can do too much. Once its 5 v 4 its game over. Things like holy nova and COH and POH aren't powerful enough to let you survive long enough. Yes, Guardian Spirit, Penance, etc will help in survivability for a while but you are then in defensive mode and not killing anything, when those cooldowns are all blown you are dead.