Article link: http://blog.askmrrobot.com/nps-robust-tanking-metric/
Yellowfive explains NPS (or as we lovingly call it "NoPeS"). It is an evolution of the other versions that have come before it, like Theck's TMI, etc. The TLDR: It tracks with death chance, so it measures your ability to survive very well.
Common criticisms revolve around the claim that you can't sim tanks. Sure, it's complicated, but you need some reliable way to determine what talents or what items might be best. Also, it may not surprise you that we believe you CAN sim tanks, and they just needed more love than they've gotten in the past.
So we set out to improve on tank simulations. The highlights: you can die (in SimC you can't), you get a heal team that reacts to your health like real healers would, and we built in boss events that you can anticipate and save cooldowns for.