Firstly, I love the style. I am a big sucker for that kind of retro FPS. I've never seen a game with this style where you can jump!
I think the pacing of the game is a little off, probably because of the balancing. It doesn't feel like I am fighting against the pressure of this countdown, it is almost like a secondary thing thrown on top. With some balancing work, I think you could make that feeling of pressure and tension a lot better. The combat mechanics were very well done though, and the art was great. Awesome job!