blizz did a patch in like feb/mar of 2009 that boosted the effects of resilience for players 70 and lower, PVP gear was pretty much BIS for 70s in the 70-79 brackets enabling you to deal with players 71-76, some 77 classes, but having issues with 78-79.
this caused huge issues for 50-59 and 60-69 (before the great twink nerf), as those twinks geared more towards BC max socket gear, which made healers unkillable
MOP killed using PVP gear, there was a base resilience for all players, anything you equipped that took you past that had huge DR to the point of being worthless. most everyone went raid, crafted, etc.