T3: Halo 4 Preview
There's no great change in visuals from Halo: Reach particularly. It's become fairly common knowledge that developers have stretched both the Xbox 360 and the PS3 to their maximum capabilities now, and that's been the case for nearly a year now. That said, the levels are well designed, with a real emphasis on anti-linearity, and there are some really beautiful vistas on show.
Most importantly, despite the fact that the build we were playing was incomplete, the frame-rate was excellent even during intense firefights and there was nothing in the way of screen tearing or graphical glitches. All in all, very impressive.
Also, some campaign spoilers in that article.