It's always interesting to see new technology for sound. There's a huge opportunity for someone to create software that's better able to understand music the way a DJ might (BPM, key, etc) and automatically create a mix by beatmatching and cycling through the circle of fifths, for example.
thisismyjam.com tries to do that. It's a demo product built on The Echo Nest APIs. It's cool to see something like this on the web, but from my testing, it's about as good as existing software out there that's desktop-based or built into hardware like the Pioneer CMX-3000.
There's no reason why a computer can't eventually do the same thing a human DJ can. In the meantime, here's a quick mix I created with thisismyjam.