Once you are 8 levels above a mob's level, it will turn gray and no longer aggro on you (i.e. attack you first). Gray mobs will still link (attack you if you attack or are attacked by a nearby like-kind mob), however. It also seems that gray mobs do very little damage when they hit you- I'm talking like 1's and 2's from what I've seen so far. And, of course, the mob won't be worth much experience, if any.