The first thing that impressed me was the awesome music selection. Really gets the blood pumping.
The enemies also have really original designs--and there are so many of them, especially the bosses! Wow.
The controls are generally smooth, and the combat is intuitive.
However, the incredibly high/fast double jump comes with a certain feeling of loss of control. It's a tricky tradeoff.
Since the area is rather large (as is to be expected for a Metroidvania game), a map would have been helpful.
Also, for some reason the fonts would not display properly for me, regardless of whether I was in fullscreen mode or not. See below for screenshot:
