As a DJ I can tell you not to book me for your club. I can do whatever but it's just whatever. I spin because I worship music. Looking for something from a DJ really depends on the situation. It's really not much different from MCing. Not all MCs move the croud; not all Djs rock the house, but you can appreciate them all. You can anyways...
So to break it down:
club djs need to keep the girls on the floor - shaking that ass
mixtape djs need to keep the new music on your stereo
scratch djs remind you that all that itching makes you scratch
the rest of those djs that aren't suckers remind you that music is good and you should open your fecking ears
Butall djs should have one foundation skill which is matching what you do to the audience that is listening.