In my guild (Covenant - Deathwing Horde) we're running with the setup I'll note below and I believe it's the best one I've had up to now.
Tanks:
- 1x Protection Paladin
- 1x Blood Death Knight
Both of them have great utility and both can help the raid in great lengths. They're also the optimal tanks for LoD encounters.
Healers:
- 1x Restoration Druid
- 1x Restoration Shaman
- 1x Holy Paladin
- 1x Discipline Priest
Four healers are usually enough, as long as they're decent. Fitting an extra healer there will just make them overheal each other. In addition, you gain an extra DPS spot for your raid setup.
Ranged DPS:
- 1-2x Balance Druid
- 3-4x Fire Mage
- 2-3x Marksmanship Hunter
- 2-3x Shadow Priest
- 1x Demonology Warlock
- 1x Elemental Shaman
The Ranged group is pretty flexible. It's always good to have an extra one of them. Regardless of which fight, Ranged usually have a better positioning than Melee. Take Lich King fight for example, whenever you have to spread for a defile, Ranged can already stand at the position where you're supposed to spread, and so forth. In addition, FFB Mages and MM Hunters can attack the Lich King in Transition phase as well.
You're also able to fill in an extra Warlock, a Hybrid or Affliction, depends on your choice.
Melee DPS:
- 1x Fury Warrior
- 1x Feral Druid
- 2-3x Retribution Paladin
- 1x Frost Death Knight
- 1x Unholy Death Knight
It's always good to have an extra Retribution Paladin. More Rets you got, that means more dispelling on the tanks in Transition phase of Lich King encounter. You also have an additional stun, extra Hand of Salvation for Mages.
Overall, the raid group can be flexible, but in this range of what I wrote is basically the best setup I've ever had. Good luck.