A difficult, yet satisfying game, with epic boss battles, great music, awesome graphics (for 2D, obviously -- personally, I love 2D the best, though, so that's actually a plus for me), and great replay value. Also, there are "advanced techniques", using Smash Bros terms. Single Wall Jumping and Infinite Bomb Jumping, just to name a few. That can add extra time, to use during your runs through the game or just to practice and learn for the heck of it. Then there's sequence-breaking (getting something before you're supposed to, or getting past an area you're not supposed to be able to yet -- for example, using a trick in Brinstar, you can get a Super Missile tank before, or without, fighting Spore Spawn to get the official item) to add even more replay value. And to top it all off, it's a Metroid game, so speed running, low percent running, and 100% running are all great options to add even more to the experience.
Plus, the environments are so diverse, creative and awesome that it only makes it better.