I love the original Metroid. It's not really sensible to review a game almost 20 years old and compare it to the standards of today (standards which suck, if you look around and the majority of titles coming out) As far as I'm concerned, Zero Mission was an amazing remake, with some very faithful arrangements of the incredible music from the original. Getting the original game built in is a plus, I think, especially since you can now save.
The password system was the only thing worth bitching about.
Oh and a little bonus troll for everyone - my personal least favorite Metroid game is Prime.
I rank even Return of Samus (a decent game!) above it.