Poor gameplay.
The game had horrible pacing. The controls were sluggish. The game was riddled with glitches, and felt a lot like a feature creep. The only saving grace is that artwork is par to what has come to be expected of flash games of the genre.
However: the break down goes as this.
The pacing needs to improve, more chaos, less controlled "obvious boss patterns". The controls must be improved as well.
Sluggish controls lent themselves to even further failure because of the distinct inability to move and attack at the same time. There also have been slight intervals between each movement that makes the game less enjoyable, it's almost as if you let the animations take priority of the game-play.
BUGS! As someone else has mentioned it's possible to have bosses that never die, or bosses that ARE dead but not counted as dead, either forcing you to restart, or find a creative way to kill yourself (with the exception of the first stage, there was none). There have also been times where the character simply got "stuck" here and there.
The feature creep comes when where you have silly mini-games that detract enjoyment from the game rather than add to it. What does collecting stars have to do other than add a meaningless and tedious task for the player to do? Or rapidly press A (with from what I saw little consequence for not tapping A)? The context is important, there was no context for the stars, and hardly any reason to be tapping the A button unless you gave the player a perceived sense of danger or urgency. Take what you did with the time-bomb for example, THAT had a sense of urgency that got me slightly hurried even when the solution was obvious.
However, the artwork in of itself was good which is why the game deserved more than a failing grade. However in gaming, game-play is king, not artistic ability. It was a good attempt and an ambitious one at that. It is unfortunate it fell short on so many aspects.
