I don't see the fun in this game, and that's coming from somebody who loves both Guitar Hero and Rock Band. I like dance, house, and other forms of sampled music, but the tracks in that demo are all crappy remixes of good songs. I also don't see the challenge -- it looks like there are three tracks and even on expert they are quite simple.
The thing that makes games like Rock Band exciting is that they feel a lot like the real deal. Sure, it's not really guitar, but it's no different than how I'd play air guitar, and that's close enough to feel good. The singing is as real as it gets and the drums are quite close (just fewer drums). All in all, you feel like you're really the star playing some epic solo.
But DJing isn't just pressing a couple of buttons in time to the music. If you had to pick out tracks, beat match, and crossfade, I could understand, but simply pressing buttons in time to the music feels like a three-finger version of Dance Dance Revolution. The only thing I could see remotely satisfying is scratching the record back and forth, but that's not a major part of most dance music. Scratching is to DJing what heavy power chords are to rock: they add flavor to the music, but there's a lot more that goes into playing the music. DJ Hero just doesn't seem to be close enough to really capture that feeling, and the choice of music seems to emphasize that.
A good dance song is more than just scratching a record over top of a classic 80s song and DJing is more than just pressing a few buttons and scratching a record. Without something a little more complicated and a little closer to the real deal, I just can't see the appeal of this game.