Reaching Crit Immunity: Resilience > Defense
Resilience seems to cost much less points on an item than Defense Rating and as such wearing a PvP items might offer more for tanking than equivalent items found in PvE which are actually intended for tanking.
It takes 60 Defense Rating to reduce your chance to be crit by 1%.
It takes 39.4 Resilience to reduce your chance to be crit by 1%.
So it takes 336 Defense Rating to get to 490 Defense and become crit immune. Where as it would take only 220.64 Resilience to create a similar effect and since Defense only adds additional avoidance when it comes to Effective Health PvP items seem to provide more Effective Health than regular PvE items.
Merciless Gladiator's Shield Wall provides 31 Resilience.
31 Resilience / 39.4 Resilience Rating = 0.78680203% reduce to crit
0.78680203 * 60 (the amount of defense rating in 1% crit reduction) = 47.2081218
...which means that this shield could be said to have ~47 Defense Rating
This is obviously pushing the envelope, but I figured it'd be a good thing to discuss.