I love pure jumping levels like these, and Zero Hour is one of the most refreshing chapters out there, but I have things to say!
A couple of the levels have palettes that are pretty hard on the eyes, especially Slope and Stepping Stones. They're not very difficult though, so one doesn't have to stare at them for too long. (Otherwise the minimalistic colors in the chapter are great.)
Also, as you pointed out, the jumping physics are really versatible, but some jumps off diagonal surfaces feel more buggy than anything. (Last jump of Stepping Stones, for example.)
The button-and-hole jumps in Yellowjacket are a really clever idea, but at least for me, there's something strange going on. When I press a button and run into the opened hole, I can't jump from the wall right away. I have to let Meat Boy slide down a bit, and the jump will fail because he hits the opposite wall of the hole. The jump is successful maybe 10% of the time, which is excruciating since you have to succeed multiple times in a row. (I doubt it's FPS related, since I have stable 60 at all times.)
The final thing is common with many a great chapter, and more of a pet peeve than anything, but wonky spike hitboxes are infuriating as all hell. Zero Hour has its share of this, with a bit of inconsistency. For example, Squeeze/Fix has those descending vertical pipes after the first jump, with a little bit of sawblade visible on the outer walls. These blades don't kill you.
However, in Stepping Stones, third to last jump is done from the leftmost sawbladey block. The block's right side is completely blank, but you still die unless you hit dead center. It just feels wrong.
And I have to ask: What's the deal with Diamond Quarter? Managing to jump off the invisible wall bit with 1 pixel center seems to be completely random, and you have to pull of a *lot* of those consecutively. Can someone finish this level consistently? (I can't seem to be able to finish it at all.)
Anyway, really nice work and design ideas, but some levels have too many unfair/random elements to be enjoyable.