Just a few things.
I have only played a few levels so I can't really write a complex review about this yet, but I wanted to share a couple points that bothered me :
-the backgrounds, while very good and detailed, do not contrast enough with what's happening in the foreground; it's not always easy to spot the enemies or the loot. Consider adding a low-opacity white-ish layer between your background and the rest to simulate air density.
-overall, the gameplay lacks energy; there should be a sound playing everytime I hit or kill an enemy, and the animations could be a little more dynamic.
-the way the rider seeks the mouse (and the loot seeks the rider) seems to be using an algorithm that makes things move only in 0° and 45° lines; it makes movements feel geometric. Here is an algorithm I like to use for that situation -
...where Speed is between 0 and 1. I usually use 0.5 for a smooth movement.
This makes the object move straight to the goal position, and slow down gradually just before reaching its target.
Hope I didn't sound arrogant or anything, I just wanted to help.
Also, I'll have to agree with everyone and say the soundtrack and graphics are just plain awesome.
Now if you'll excuse me, I'm going to play the rest of this game !