Metroid: Zero Mission is a very good game, but it's hampered severely by it's length.
The story here is rather simple. Samus Aryan is a intergalactic bounty hunter charged with the destruction of the Metroid, a creature with the ability to siphon the life out of any living being. After being stolen by the evil Space Pirates, the creature was hidden away on the Pirate's home base, Planet Zebes. Samus has to destroy the monster and those who have taken possession of it so as to ensure that they are not able to use it as a weapon against the rest of the universe.
Metroid is essentially a free-roaming two-dimensional platformer. You are placed at a starting point in a rather large world and given the freedom to wander around it as you see fit. At least that's how it was in it's original incarnation. In this game, you are given the aid of a hint system that more or less tells you exactly where to go, which may well disappoint people who like to figure these things out themselves, especially older fans of the series. It makes the game seem just a bit too easy to be told where to go instead of figuring it all out yourself, and given the game's relative length, that can be a very bad thing.
The rest of the gameplay remains pretty much the same. Everything controls and plays the way that it did in Super Metroid, or, more recently, in Metroid Fusion. You shoot enemies and open bubble doors to find new areas and so forth. Collecting upgrades is the only way to reach new areas, most of these are items that have been seen in older games in the series. The few new ones don't feel like major additions either, as they only give you the ability to do a few new things that eventually become almost entirely unnecessary anyway, like grabbing onto ledges and hanging from them. Eventually, after you collect most of your upgrades and find things like more missiles and energy tanks, the game appears to become a lot easier. Too easy, in fact. The difficulty level doesn't really seem to rise that much in comparison with your own powers, which makes the ending feel like a bit of a letdown.
The game looks fine graphically. Compared to Metroid Fusion, everything looks a bit on the cartoony side. It feels as if there's slightly less detail in everything. The game's colors all look a lot more vivid than they did before though, which helps it's appearance quite a bit. There aren't any glaring flaws in the visuals either, and all together everything here looks quite nice. Everything animates just as well as one could ask, and you can't blame what you see for anything that goes wrong during gameplay.
The sound in this game comes up to a very high standard. The music consists of several tunes from the original version of the game that have been remixed for the Game Boy Advance. These all sound quite nice and can be appreciated by anyone familiar with the series. Sound effects meet a high standard too, there's nothing to complain about with them either. You'll hear just about everything that you could expect from a Metroid game.
Metroid: Zero Mission is an otherwise fine game that's hurt severely by it's length and it's difficulty. It's very easy to finish the game in around two hours with a little practice. Not only that, but once you've done that all you have is to either play it on a higher difficulty setting, which doesn't increase the difficulty that much, or try out the NES Metroid, which is a fun game itself but feels a bit outdated these days. It's a good quick pick-up, worth any GBA owner's time, but it just doesn't feel nearly substantial enough. It's too short and too easy to meet the standards of other games in the series perfectly.