Actually 2 of our Fire-spec'd Mages beat 2 of our best Rogues last night in DPS in AQ40.
Druids are completely tamed into healers endgame. While some may contest that they do decent DPS in feralform, they don't really compare to a Mage when it comes to spellcasting and mana-efficiency, nor do they crit as high as a Rogue, DPS Warrior or Hunter in feral form. Their heals are rediculously powerful however. I've seen 7.6k+ crit heals by Druids before.
Endgame, there's not a lot Mages can polymorph, pretty much everything is immune, or non-humanoid, the exception being Majordomo Executus's guards. And starting out in MC/Ony, you're pretty much just spamming the Frostbolt/Conjure Water button from beginning to end.
Priests obviously just spamming heals. In AQ40, they're needed to Mana Burn certain mobs that will blow up the entire raid if it aquires full mana however. It starts out with 0, and sucks a little bit from everyone in the raid. Our guild did recruit a Shadow Priest for extra DPS, but he was usually below all of the Mages and Rogues. In the end it was more worth it to just recruit another Rogue or Mage.
I've never really noticed the role of Warlocks, though it seems they're mostly used for DoTs and Curse of Elements/Curse of Shadow. Typically the Warlocks that I see trying to go all-out with DPS usually rip agro off the tanks, and die, so while they may try at first to do as much damage as they can, you'll see them start to calm down as their repair bill starts to hit their wallet.
Don't really feel like finishing the rest of this right now though. =(