This was pretty cool. I also dig the 3D/2D artstyle. That being said, I only have a few nitpicks/suggestions:
- I assume this is already planned, but if it isn't, please add the option to adjust the mouse sensitivity. Hair-trigger aiming isn't for everyone, so the ability to customize it and tone it down would be greatly appreciated by many players, including myself. Also, customizable controls in general would be awesome.
- The Headcrab Zombie's attack range seems a little large. I got swiped by one even though it looked as if his attack should have missed. Also, the frequency in which the enemies can attack can be a pain to fight against sometimes, especially if you get slapped into a wall.
- It kinda sucks that you can't climb onto movable boxes, but if it's simply not possible due to the engine/physics, then I can understand that.
- While on the subject of movable objects, the ability to simply place something down in front of you instead of chucking it full force would be nice.
Other than that, this was a fun little demo, and I'm looking forward to playing more. Keep up the good work!