If you're interested in PvE... dungeons and the like... you'll want a tank capable character and a healer capable character on your team.

Death Knights, Druids, Paladins and Warriors can tank; even if your team has a Boomkin (Balance Druid) on it, that character can queue as a "tank".
Druids, Paladins, Priests and Shamans have healer specs; having one of these on the team (in addition to the tank toon) lets your queue for dungeons with the Looking for Dungeon tool.

If you're serious about PvE, you'll progress to heroics.
And its good to have the option of a legitimate tanking class.
You can use Hunter or Warlock pets for normal versions of the dungeons, but probably not for heroics.

With RAF, I'd say get a bunch of combinations.
I personally enjoy:
- 1x Paladin & 4x DK's
- 5x Druids


I've not finished leveling, but have enjoyed:
- 1x Priest & 4x Warlocks (PvP)
- 5x Priests
- 1x Druid (Balance spec1 and Feral spec2), 3x Elemental Shaman, 1x Resto Shaman
- 5x Holy Paladins (Smite/Exorcism dps build)