Solid controls. Cute, well-animated player character. Good music and sound effects. Enjoyable progression of abilities. I also liked the countdown segment toward the beginning--very faithful to the roots of the genre! :)
The environment is somewhat generic--many rooms do not strongly suggest the "underground" theme.
Also, I believe there may be an error with the way the HUD displays the player character's health. I often had 0 hearts of health, but I wouldn't die unless I received another couple of hits; I think this means there was an extra invisible heart somewhere.