Valithria Dreamwalker

The Dragon Who Loved Me.
from the memories of an Icecrown raid healer

Ok, that's a completely silly and cheesy subtitle, but I can't help. That's probably the first fight in the history of WoW that puts healers in the center of the stage. Yes, during that fight the goal is not to kill something, but to heal a captured green dragon.
My healer friends, we will have to bring back this mighty dragon from 50% life to full while our raiding brethren will protect us from the enemies assaulting us and trying to prevent our task.
Isn't that so cool to be the one that will decide on the length of the fight ? Yes you've read correctly this fight is not a DPS race, it's a ... Healing Race !

Let's come to more details. Valithria Dreamwalker is the first boss of the Frostwing Halls of Icecrown Citadel and is encountered before Sindragosa. She starts the fight at 50% life, you will need to heal her while killing the adds that will spawn to stop you. Once the dragon is back to full health she will destroy all remaining adds and teleport her (and you) out of the citadel effectively ending the fight.
During the fight players can use portals created by Valithria to switch to dreamstate, and alternate plane where dreamclouds will spawn. These clouds will regenerate your mana and greatly increase your healing for a short time, you can then get out and bomb heal Valithria before going back in there. While some healers (and a few dps as the clouds have to be destroyed to give buffs) will rotate between healing & regaining mana, the rest of the raid will have to deal with all the adds.
There are 5 kinds of adds. Glutonous Abominations will be especially dangerous as they'll spawn Rot Worms when they die (8-10 in 25-Man) that will melee their target (for about 5K) and can easily OS any character that is not a tank if they gang on him. But the key of the fight will be Suppressers, a class of adds that cast a strong debuff on Valithria reducing all heals on her by 10% and effectively making the fight longer as long as they're up. In 25-Man there is 6 Suppressers per wave when this fight was tested on the PTR.
You can find some details about the abilities on the wowwiki page, or the MMO Champion page (with a nice video of the fight).

Now about tactics, my guess will be for the Valithria healing squad to switch to dream state when a wave of adds spawn, buff and regen there until the Suppressers have been killed by the raid and come back to heal Valithria with full mana and the healing bonus while not being hindered by the Suppressers healing debuff on the dragon.
Now for the classes, I think Holy Paladins and Restoration Druids will probably be the healers assigned to Valithria if you have enough of them, while Shamans and Priests will probably stay on the raid. Why ? Because their single target spam healing is probably the best of the 4 healing specs, though very mana intensive, hence the use of the dreamstate plane. Holy Light spam and Nourish spam (whith full hots active) will be hard to best.

It really looks like a very interesting fight and I'm really eager to try it, especially in 25-man where the number of adds can turn the fight from controlled to hectic in a few seconds ^^