I've been running a priest and it has been "doable", but there are some fights that are pretty rough.
Monk has been pretty solid.
I have messed with Holy Pal + 4DK instead of Prot + 4DK ... DKs are still undergeared, but it's doable so far.
Druid multi-box healers when setup right are a massive difference, their mobility is too good.

Right now my trinity comp that I would recommend to people trying to find a group:
DK/DRU/BMx3 run as many undead toons as possible for the fear break :P I've been using trolls for the haste until late.

2nd choice for Tank is Paladin
2nd choice for Healer is Monk
2nd choice for DPS is Warlocks