Resil = Decrease chance of receiving Crits.
Def Rating = 1 point of Def Increases the following by 0.04% (warriors)...
- Increase chance to block
- Increase chance to dodge
- Increase chance to parry
- Increase chance to be missed
- Decrease chance to be crit
- Decrease chance to be dazed

Critical Strikes is the first thing to be removed from the hit table if your avoidance is high enough (490Def for warriors, lower for feral druids).
Meaning by 490 def, you will be crit immune (but still crushable).

that is why point per point, DR gives u more bang for your points.

edited for greater emphasis on resil.

39.4 resil = 1% chance less likely to receive crits.

490 def is crit immune to mobs 3 levels higher than you (level 73 bosses).
So if you're PVPing with people of same level, you don't need 490 to be crit immune.