Right so I played the demo, and here are my thoughts.
- The controls are very nicely tuned. You nailed this aspect.
- Sound is good, although I'm not personally a fan of the music that much.
- The enemies are well programmed for the most part.
- The Level Design. Oh man, the Level Design. The first section is far too empty and kind of dull, with not much in the way of obstacles or difficulty. Then the second part hits us with a ton of bricks in the face, requiring far more precision than any section of the first part. Furthermore, some of the (I assume are) bugs make this section even more tedious than it needs to be. And that last little jump at the end? I know that you were going for a flashy finish and all, but having to restart the whole second section when you're right next to the boss door due to not knowing about the jump is just not cool.
- I was stuck in a jump animation while landing a few times. Really minor compared to some of the other stuff, but I figured you would want to know.
- The wall shooters can get frozen in place if you back off screen while it's turret turns.
- The damage barrier does nothing against damage surfaces. Seriously?
- I killed the boss as I was about to be hit, and died as he was exploding. This led to me being frozen in place with no sound when I respawned at the checkpoint. I was not particularly inclined to go through the level again (see Level Design), so I don't know what it's like after that.
I like the controls and enemies. The music is not for me, but I can appreciate the sound design. The Level Design was not that great as far as I'm concerned, some tuning to equalize the difficulty/enjoyment across the board is definitely needed.