The challenge (well, until you beat the games enough times… 
) and the stories 
Also, it has alot of cool upgrades, bosses, and some pretty good/cool background music.
Oh, and like TDJ said, the area’s are cool too! And detailed!
[hr][/hr]
OH, one of Got Metroid???'s posts reminded me of something else!
I also like how the creatures/aliens in the game ACTUALLY look REAL, and not just like some random globs with tentacles! ( 
)