First off: Excellent. Smooth as silk gameplay. The sprint mechanic is great and really helps you play with vertical puzzles/secret-finding. Bullet time worked very well and felt great. My only suggestions are small, personal opinions for tweaks.
1. Maybe increase the delay between when the yellow zombies see the player and when they fire. If you are too close when first spotted, it's nearly imposible to dodge.
2. It was sometimes unclear where to go with the keys - the lighted signs were great, but some more environmental cues saying "this is a key door" might help. Maybe I'm just blind, though.
3. Guns maybe feeling a little weak; this is probably a purely psychological/sound issue, since hits-per-kill I think they're pretty perfect.
4. I found myself not using the thrown katana, since if I was out of ranged weapons, only being able to use it ever 5 seconds or so was too slow.