Mitigation and Damage
Warriors take the least amount of damage out of any tank, and deal the most damage. They also bring Spell Reflect which, by itself, speeds up certain packs/bosses tremendously.
Damage Buff
Warriors bring Battle Shout! While scrolls do exist, you get 3% more Attack Power with Battle Shout, which can make a difference on higher keys.
Group Utility
Warriors bring Battle Cry - a group defensive cooldown - as well as Intercept and an AoE fear in Intimidating Shout.
Mitigation and Damage
Similar to Warriors, BrM Monks take a fair bit of damage, but are able to Stagger it. This lets healers have a much easier time and smooths out incoming damage. They also do a large amount of damage.
Increase Enemy Damage Taken
Monks bring Mystic Touch! This increases your overall damage by 5%. Unlike warriors, you cannot buy this in a scroll, so Monks are amazing to have for damage purposes.
Tons of Utility
Monks have insane utility in the form of Ring of Peace, Leg Sweep, Detox, an AoE taunt/speed taunt with Black Ox and Provoke respectively, as well as Paralysis and Transcendance to setup certain pulls.
Group Healing
Paladins do not have nearly as much mitigation as Warriors and Monks. However, they make up for it in healing. Not only can they heal themselves, but they help heal the party as well.
Interrupts Galore
What's one of Hunter's weaknesses? A ranged interrupt. Lucky for us, Prot Paladins bring a million interrupts. Between their standard Rebuke and the resets they get from Avenger's Shield procs, you should always have enough interrupts.
Group Utiilty
Paladins bring an insane amount of group utility between Freedom, Blessing of Protection (or Spell Warding), Hammer of Justice and Blinding Light. They also have unique abilities such as Divine Shield which lets them immune abilities and ignore certain affixes such as Necrotic.