Simple but fun concept. I like the panic to quickly move my hand over the right keys (good idea to always keep the classic WASD alignment to keep this more immediate). I would have like a quick feedback just before it was going to switch, in order to anticipate the change (would still need to react quickly and reach the right keys after).
Not sure having the physic be different for slopes was worth it (you kind of lose control a little bit when reaching the top of the slope, and enemies go too fast when going down a slope in my opinion).
Good job anyway :)