Tanks: Pala, DK, mby Feral bear [in ToC, people will need pala and warr with block gear]
Healers: any (except mby holy priest; that spec is mostly good on XT hm)
DPS: Ranged (especially warlocks, spriests, hunters), Rogues. The reason for the preference of the rdps is because most bosses benefit them a lot.
On Hodir, ranged deal extra damage thx to the flame debuffs (I think that dks and rogues also benefit from them).
On Thorim, you want to have as few melees as possible due to the Chain Lightnings.
On Iron Council, you need a hunter with a brain to get the buff and deal extra damage to the boss.
On Freya, you need either a boomie or an ele shaman to typhoon the detonaning lashers before they die and a hunter to kite the snaplasher.
On Vezax, you mostly need ranged dps. Mdps deal less damage than them and are only good as kickers.
On Algalon, you need a shadow priest for disperse (it's also possible to make a tank use defensives for the Big Bangs, but that person would be quite vulnerable to Algalon's attacks).