Maybe adding a select level screen, or something so i don't have to replay the first hardcore mirror level every time i exit. If you add a button to turn out music would really improve the game experience (for me XD, in this case)
Furthermore i consider some of the hardcore levels just annoying, instead of challenging, however i loved their dificulty, great game overall ;)
Finally, some achievements will increase the acomplishment sensation, but that is a personal opinion