In terms of the concept, I think this was very innovative and I personally enjoyed being able to play the enemies of the plumber and kill him. As this is much like Mario, it was nice to finally give the little plumber a taste of his own medicine from the villain side. That, the jovial soundtrack, and the lovely look of the levels was what gave you four stars.
What took away from five was the absolute tediousness of the levels as a whole. Most of the levels were okay or just difficult enough to be entertaining, but some took it to a new level and made it EXTREMELY hard to progress, enough to make me wanna rage quit. By the time I got to the lost levels, I honestly stopped playing because they were optional and much harder than the level as a whole.
I've seen this a lot of times, and I think this is some good advice: It's great to make the game hard and make players work for it, but to many people there's a limit. Once the limit is reached, nobody will desire to play it anymore and you'll lose some of your fanbase. I've seen this happen with a lot of games (Although I'm usually the one that keeps trudging on), where the puzzles/levels are just impossibly hard and people eventually quit out of frustration. Maybe that's just me.
Anyhow, I'm going to be checking out your sequel now with hopes high that it will be an improvement so I can give you the five stars I feel you deserve.