It's hard to tell what to do from within the game; the only way to know that there are 4 keys, rather than 3 or 6, is reading the instructions. Movement was a bit slow without sprinting and the mouse was running into the screen edges the first time I played it.
I don't mind having the cubes look like the rest of the objects; I would recommend that they be put a bit further away, to make the player have to move them to get the keys (and thereby realize that they can be moved).