Damage Mitigation and Tanking

Critical hit immunity at level 80 for a heroic dungeon is 535 Defense.
Critical hit immunity at level 80 for a raid boss occurs at 540 Defense.

According to WoWWiki, as of patch 3.0, crushing blows cannot happen to max-level tanks. They changed crushing blows from 4 levels below the boss level (83) to 3 levels below the boss level. This change was to improve the validity of the druid and death knight tanks.

Damage Mitigation:
Block - Additional chance to mitigate damage when wearing a shield.
Defense - Helps prevent physical damage by reducing the chance to be hit or critically hit. It also increases block, dodge and parry.
Dodge - Ability to avoid an entire attack.
Miss - A failed attack. (Defense Skill vs Weapon Skill)
Parry - Chance to nullify an attack.
Resilience - Reduces chance to be critically hit. Reduces the damage from melee and spell critical strikes that do hit. Reduces normal damage taken.
Resistance - Reduces, or entirely blocks a particular shcool of magic.

Warrior
Warriors are best for tanking melee bosses with a lot of attack speed and low damage per hit.
They don't do well against spell damage.
They are able to use all the damage mitigation abilities.

Paladin
Paladins are best for tanking bosses with multiple adds.
They can also use all the damage mitigation abilities, but are more reliant on reaching the block cap for shields.

Druid
Druids are best at tanking burst damage bosses because they have huge health pools.
Cannot wear shields = Cannot block.
Weapons do not deal damage when in bear form = Cannot parry.
Depend heavily on high amounts of dodge from agility.

Death Knight
Death Knights are best for tanking spell-casting bosses. They have several abilities that enable them to avoid spell damage all together.
Receives 25% of strength as parry rating!
Cannot wear shields = Cannot block.