A faithful remake of an extraordinary game.
Gameplay (8/10): No matter what happens, this is the original Metroid at heart. That being said, the basic gameplay holds up quite well. The player's hand is held a little more than it was in the original (which wasn't held at all), in order to nudge the player in the right direction, but if you are creative, there are ways to get around this. My main complaint about this game is that the complete freedom of exploration is lacking in this game, you are generally forced to go to certain areas in a certain order. Still, there are tons more upgrades to find and half of them are a lot more challenging and creatively placed than the original, so that makes it quite fun to go around trying out your new powers. Speaking of new powers, not only do you have access to all of the old upgrades, but there are a handful of brand-new upgrades that weren't in the original (such as the Power Bomb and Plasma Beam). All in all the new and old features combined make this game great fun to play. The three new mini-bosses are a nice way to change things up a little so that you don't get bored of going through a long stretch of normal areas before getting to one out of the two main bosses. As far as controls, this game is spot-on. The Super Metroid/Metroid Fusion control additions make this game accessible and more interesting as well. Granted, the game is not nearly as hard as the original, but thanks to a couple of the bosses, it still has it's challenges.
As far as story, well it has been beefed up quite a bit from the original "kill Metroids, defeat Mother Brain, save the galaxy" story. Now we have some backstory as to the Space Pirates and also the Galactic Federation as well as a better understanding of Samus. There has even been added a whole new area after you beat Mother Brain that helps explain a little more about Samus while giving you a little more fun before the game is over.
Graphics (9/10): The graphics are wonderful. The cel-shaded look of animation looks beautiful and really does this game justice. The environments are well made as well and the few cutscenes there are look great (though they could have been a little more animated and there could have been some more). Overall this game looks wonderful.
Sound (10/10): As if the music wasn't great in the original, this game takes it a step further with top notch remixes of the original tracks, along with some new ones to fit in with the new areas and to add variety to the boss battles. But music isn't all that makes a game sound good. The sound effects are great as well, the monsters sound excellent (hearing Ridley's screech still sends shivers down my spine) and the weapons sound strangely satisfying to use. There are even varied footstep sounds depending on what kind of surface you are running on. This is aural bliss.
Value (8/10): No matter how fun this game is, it still suffers the same problem that the original did...it is too short. Even if you are not that good, you will likely breeze through this game in 3-3.5 hours the first time (I generally take no more than 1.5 hours now). Still, why does it get an 8? First of all it is loads of fun while it lasts. Also there are a bunch of extras. There is the unlockable Hard Mode which is actually very challenging and fun to play through again. There is a photo gallery. You can only unlock all the photos by beating the game in certain ways. I admit it is a very lame bonus, but it at least gives you an excuse to accomplish all the goals. There is also the (fairly) useless feature of linking up with Metroid Fusion to be able to see all the pictures from that game on MZM as well. Lastly, you can unlock the full original Metroid after you beat the game.
Really, even though the game is so short, it is worth the money because of the huge number of upgrades to find, the challenge of Hard Mode and the ability to play the original Metroid wherever you want (and you don't have to pay $20 for a GBA port cartridge), it even lets you save your progress after you die and you can still use your old NES passwords as well.
Nintendo doesn't often remake a game (they rerelease with a few new features, but rarely remake), but when they do, they put in their best effort and usually end up with a wonderfully polished product like Metroid Zero Mission. If you own a GBA, GBASP, GBAM, GBAP, DS or DSL and you don't mind action/adventure games, then this game is definately worth a buy. Enjoy.