Well thought out level design, great artwork, mechanics definitely could use some polish. I do think that some variation in music throughout the levels would be nice, as well.
Loved level 15.
*Since I saw a bunch of people were having issues with level 15, skip if you don't want to be spoiled: lower left door, get ice, then the other two keys, go back the way you came, get tornado, then get fire/earth, go to the edge of the platform on the bottom, tornado back up, go to the upper part, use one lever, and then jump through the flames using earth mode*.