It’s not easy to get the health bars of every raider near the center of the screen without obscuring the entire view area. In retail every decent healer streamer has them near the character but with clear vision of the ground, but it’s a little tricky with 40 instead of 20.
—Health bars of every raider
—Mana bars of every raider
—Any HoTs that are overwriteable such as renew you should be able to see so you’re not overwriting
—Debuffs that are important should be color coded
—Ability to turn on and off specific debuffs
--Edit: Range finder / graying out people out of range
--Edit: Predictive healing, very important in such large raids with such slow casting heals.
—Ability to see someone missing buffs you can give (edit: separate addon for this is fine)
—Your own health and mana bar near the center of the screen
Not as critical but not a bad idea:
—Healing meters (good for catching mistakes, for example one day I didn’t swap a mouseover macro back from my benediction quest)
--Threat meters
...and then there’s still a lot more that can be done, but it starts to get in to WeakAuras for specific fights. That level of UI design isn't really needed in Molten Core, but I'm guessing by AQ40 some of that can be helpful.
Oh and do we have EXRT yet? I still need an addon for raid cooldowns.