There's been a lot of talk regarding fear, I believe. At the moment it can't be implemented because there isn't any pathing during / agfter pulls. Thus, a feared mob would run in a straight line away from you, most likely into a wall.
Some people have suggested stunning the mobs (instead of having them run away) but there are too many problems with that. There's a big difference between a mob stunned for 2 minutes and a mob pathing through a group of his buddies for a few minutes.
I did get the impression, however, that fear will be a possibility with the release of 0.5.0 and that's just another reason to look forward to it.