Kirby frequently tries something different. In fact it may just be the most experimental series to date. There is a puzzle game, a racing game, the interesting mechanics of Canvas Curse and it's spiritual successor, Mass Attack. There is even a (really fun) golf game.
It's the alterations between Kirby's main platformer series that gets attention, and Amazing Mirror fits this bill. Going for a Metroidvania feel, Kirby and the Amazing Mirror is a blast to play for the same reasons as Metroid and the newer Castlevania's. However, by throwing in Kirby's abilities and puzzles relating to said abilities, you have a game that, I think, outperforms them in every way.
Now if it only was on something else other than GBA...
Score: 9.0 (Fantastic!)