I will list here problems which I noticed at first:
1. I don't see point in using mouse if there no aim anyway.
2. Also death animation of first alien isn't fun.
3. You never know where you allowed to fall and where you not (if you always go right since first platform you will lose).
4. Some parts of levels look like ground, but the other ones with same texture look like simple rectangles.
5. It could have some check points. Waiting on elevators is really annoying since you can fall and die somewhere.
7. Particle effects?
8. It's 2014 so maybe any kind ragdolls and physics? Or at least animations.
9. You can jump in mid-air if you step off platform without jump?
As for me fun / time ratio is too low. I'm kind of players who rate gameplay before scenario. And fun from gameplay is something what this game really lacks.
Can't tell more as I died 2 times in row at the same place. At first - because I didn't noticed platform which is hiding at the right end of visible screen, and second time - when I finally noticed it when I was falling just to restart game.