In a nutshell, a great DJ is one with experience. Yes, it can be argued that even DJ's that have 20 years of work, may not be so good. Its true. It depends on the client needs of course. Does the client want a cheesy emcee, a club DJ, a wedding DJ, a corporate DJ?
The best of the best is one who can cover practically every gamut of music, sound, and video. Now, some disc jockeys can do most types of events, but not necessarily all. How many Deejays do you know who can spin, Indian, arabic, Greek, French, and other forms of world music? There aren't too many. But, those Deejays are very specific, rare, and are unable to shift beyond their own comfort zones.
I'd argue that deejays like (examples) Tiesto, Z trip, Q-Bert probably can't spin a good wedding, or corporate event. Why? They (like their ethnic music counterparts) are very focused on genre and style. Tiesto is an electronica dj. You probably wont hear him spin funk, disco, hip hop. Z trip spins old school and new school. Q Bert is a turntablist. His style is to mix short sets of scratched tracks. Does anyone want to hear scratching at a wedding or a corporate party? Sorry Tiesto, Z and QBert for using your names. But, its to make a point.
When it comes to a great party. Make sure to choose the RIGHT DJ, with the right music, and sound.