As a level 80 Protection Paladin, I have to tank dungeons, heroics, and raids. Inorder to survive, you need to have good health, armour and defense to withstand a hit. At level 80 heroic bosses hit for around 4000 (61.04% dmg reducton so would be a bitch for mages with 15%) and about 10000 when a crit occurs. When you reach level 80, you should try to hit for 540 defense as 540 is the max you need to stop a crit. 539 would be like 2 % crit for boss but in wow some bastards dont let you in party as it might get you whole group killed.
Example one. Boss hits you for 5 k then again for 5 k. Healer heals for 4 k and 4 k again. This goes on for 15 sec but boss scores a crit and healer has to heal twice as hard and if he noob, your whole group is dead as none of other group has stats to take 8 k without going EEEEEEEEEEKKKK.
You can hit 540 def by asking blacksmiths to make Cobalt (lvl 71 def stuff) or Saronite (lvl 78 def stuff) and in the end Titanstell ( lvl 80 piec def stuff).
Defense lessens chance of boss or creep critting you. 540 is so boss cannot crit.
Also defense doesnt count when you have you back facing boss as youtake dmg accoriding to armour reducing it. Also cant block parry or dodge attacks from behind.
