The demo completely rocked my socks off. I hope it does well at retail (I've always had my doubts). I was anticipating this more than Halo 3, and now I know why:
1) It's scary as hell. You always feel paranoid. The shadows contribute to it greatly (ie you are in a bathroom, walking out, and you catch see the shadow of a running humanoid. You come out and it's nowhere to be found. And it was unscripted). The music is also great.
2) Awesome graphics (the lighting! the water!)
3) Cool powers. In the demo you can set anyone on fire, or shock them.
4) You can hax0r devices. They then join you.
5) Enemies fight themselves if they are different.
6) There's stuff to look at everywhere in the city, which provide great background to the city. This sort of attention to detail make the location feel real. Which is tremendous.
7) Objectivism is depicted as a failure, which irritates hardcore objectivists. And if there's something funnier than irritated objectivists, I don't know what funny means.
