The hate comes from the fact that up until now we've had nothing but false expectations given to us by Sony and Guerilla themselves. Now that something truly indicative has come about, it's left a bitter taste in my mouth and I imagine other people's as well.
Also, people expected Killzone 2 to look amazing. The first sold mostly on the fact that it was a good looking game for the PS2, despite the fact that in gameplay it was boring and unimaginative. If we're being told to expect amazing things then the simple fact is that they have to deliver on their promises. Killzone 1 sucked, so they have to do some pretty amazing things to make the sequel at least a semi-decent game.