I have run priest | 3x mage | tank for the majority of this expansion. I have both a druid tank and paladin tank and have tried tanking as prot & ret pally and as bear & moonkin druid. It is a very strong PvE team any way you slice it.

I've healed with a priest since I solo raided when ZG came out in the 60 days. Always been my main toon and I think there is about 100 advantages priests have over other healing classes. That being said I currently have a 42 shaman I am leveling with the intention of having another healer option. Mainly for one reason, the most overpowered ability in the game - heroism. I ran a friends shaman through a heroic and on fights he used heroism I had no problem doing 10-12k DPS on my mages.

Sadly dropping 1 mage for a moonkin increases total group DPS even though anyone can tell you moonkin DPS sucks right now. That 13% spellpower buff is invaluable for the other two mages plus the druid gives you an extra buff, but is too hard to control. Sooo... I'm picking up a DK when my shaman hits 58 and I'm going to level it with the intent of having a 3rd tank option. The big thing I'm looking at with a DK tank is they can apply the 13% spell damage buff AoE. They are the only class that can do this. That means some serious AoE firepower. Plus combine 3 mages with a tank that can do 13% spellpower buff and I don't need the moonkin which is another few thousand DPS i'll gain.

My ultimate group will hopefully be 80 soon. DK tank | 3 arcane Mages | Resto Shaman. I'll be switching the shaman to elemental as soon as him and the DK are geared. With the 13% aoe spellpower buff and heroism / totems I look forward to one of the highest DPS heroic groups ever. I have no doubt I'll be pushing 40-45k DPS with this team on heroic bosses.