Most MMOs don't have collision detection for a couple of reasons. For starters, it's a strain on bandwidth if the world has to constantly calculate where two characters intersect each other. Instead, it's much easier to just let players walk through one another. This also makes battles more manageable and keeps people from doing jerk-ish things like clogging up the doorway into an auction house.

Age of Conan is an exception. Because it has player-vs.-player combat, and because it uses a melee model based on actually calculating a weapon colliding with its target, a character's position matters in relation to other characters.

Hence this.

(Thanks to flyinj for for making our morning with this one!)