Damn this was pretty darn challenging, but quite enjoyable!
The one thing i could think of could use some sort of adjusting would be the level size or size of everything in general, maybe making the levels smaller all the while the sprites bigger sort of deal, or maybe making a dynamic camera which follows the player along - why? Because its a little difficult to tell everything apart at some occasions.
But now that i think about it making the screen size adjustable could be very simple alternative too.
But other than that, very nice work!