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.

The Ideal 5-Man group Make-up

I love this group make-up for 5-man instnaces and here's why:

Warrior Tank
Warriors were made to tank. It was an afterthought that possibly paladins and druids should be able to tank too. If you had to swap someone, you could change out the warrior for a paladin tank, if they were awesome. That way, the pally could hand out Kings or Wisdom to the party which would all benifit immensely.

Priest Heals
The priest is the only true-blood healer in the game. There are very few things that a resto shaman, resto druid, or holy paladin can offer that a priest can't. While druid heals offer more HoT's, the priest's Prayer of mending and renew are usually a much more efficient use of mana. Throw in Fortitude and Spirit which would be immensely useful for all other players in the party.

Mage
Not to mention the heavy DPS, polymorph is probably the best CC in the game. Arcane intellect would be beneficial to every other person in the party. The mage has the most consistant AOE damage of any class.

Shaman
Totems are freaking awesome. Tremmor totem to mass-dispel fear, mana regen, windfury, and a fire nova totem for AOE. I'd prefer enhancement here as the DPS is just amazing.

Druid Moonkin
Throw in a battle rez, innervate, and a 5% crit aura. Very nice spike damage and AOE capability here as well.

It also helps that having a pally, priest, shaman and a druid means that you have 4 people who can throw out an extra heal / rez if needed. If you consider their common off-spec, this is a pretty versitile group if things needed to be switched around in a pinch.

You could easily have:

Druid Tank
Pally DPS
Shammy Healer
Mage DPS
Priest DPS