Daze, he says, is based on a character's base defense rating as compared to a mob's base attack skill. The standard Daze rating is 20% (so if you're facing a mob of equal level and your defense rating hasn't been increased, then a hit from behind should daze you 1/5 of the time), and it goes up or down from there from 0 to 40%. Characters below level 30, interestingly enough, have a much reduced chance to be dazed (it's almost impossible for a level 1 character to be dazed by an equal level mob), in order to make the game easier when players first begin playing.
The reason Drysc gives for us seeing it more in Outland is a little shaky-- he says there weren't many actual level 60 mobs in Azeroth, and there are now quite a few level 70 (and higher) mobs in Outland, which means a fully leveled character will be dazed more. Finally, the reason, he says, for Daze in the first place is that they don't want to make it "safe" to randomly run through a group of mobs. "A cautious and alert player," he says, "is less likely to be dazed than one who is haphazard and careless."
My only problem with that is that I get dazed more not when I'm trying to run away from characters (I understand I'm going to be penalized when I sloppily run through a group of mobs), but when I get jumped while fighting. Should the same penalty apply when a single add jumps me from behind? And while we're at it, shouldn't being mounted lower the chance of getting dazed as well? Having a flying mount has made serious dazing tragedies much less common, but for characters still on ground mounts, shouldn't being mounted protect them a little more than being on foot?