Man, the level I just worked on in Black feels absolutely huge. I know they're probably recycling things like tree models and ground textures like mad, but what an impressive illusion of a huge space.
Also, throwing me into a forest without any sort of map is interesting, I suppose. Disorienting, especially since the number of instantly recognizeable landmarks is very low.
Dulling of senses as closer to death is a neat effect. Sort of like Max Payne but in reverse.
Also, the nifty depth of field effect when you reload reminds me of how 3dfx said their T-buffer effects were the wave of the future. They were right, but a bit too early.