Metroid Zero Mission is a remake of the original Metroid from the NES that was released some years after Metroid Fusion. Aside from the first few rooms and the boss fight against Mother Brain however, there really isn't all that much noticable reassemblence to the original NES Metroid. This is especially true since you are given almost all the weapons from Super Metroid, which in turn change the flow of the game quite a bit compared to the NES original.
In terms of overall game mechanics it basically follows traditional Metroid conventions and plays for most part rather similar to Metroid Fusion, where it diverts however is in its story telling or lack there off. While Fusion could get a little to talky at times, Zero Mission is almost void of any dialog. For storytelling the game features a series of cutscenes that work with images instead of in-game graphics. While this sounds fine in theory, these cutscenes however lack any kind atmosphere and the editing is rather terrible, thus doing far more harm then good to the overall game. The art style also diverts from more traditional pixel art, to what looks more like a comicbook style, that seems unfit for the atmosphere of a traditional Metroid game. This graphic style is not only used for the cutscenes, but also to a lesser extend visible in game. Comparable suspensful in-game sequences to Metroid Fusions SR-X encounters are missing in this game, even so there would have been plenty of opportunities to integrate similar stuff.
Instead of dialog sequences that guide you through Metroid Fusion, this game guides you mostly by statues that highlight a point on your map where you have to go to. This leads to a much more fluent open game experince, then Metroid Fusion as you are not constantly stopped for the next briefing, it however also means you don't really have any context as to why you should go to that point.
The game contains some technical improvements over Metroid Fusion, such as a new graphical effect with which tilemaps will fade away when you go below them. It is a neat little effect that is used cleverly in a few places to hide items and other mechanisms. The map screen now gives you an overview map over all the sectors and lets you switch between sectors without having you standing in them first, this saves a bit of useless running around. The game also by default shows you the items you have collected per sector, so it is easy to see where you have overlooked stuff and even without a walkthrough it is not that hard to collect most items.
Boss fights in this game are rarer then in Metroid Fusion and also less interesting. Basically the game contains just Mother Brain, Kraid, Ridley, Acid Worm and a Ridley robot. Ridley is just as in Metroid Fusion trivial to defeat with spamming it with missles, while Kraid looks almost exactly like the SNES version. The remaining ones aren't all that interesting to fight or look at either.
The most interesting and also the most broken part of the game is the zero suit section. This section takes place after you defeated Mother Brain and thus after the end of the orignial NES game. In that section you are attacked after take off and crashland back on the planet, with your suit destroyed. This means you have to fight your way through a whole bunch of space pirates, armed with only a small freeze gun, before you can get your suit back. In theory that sounds like an interesting and suspenseful change of pace for the game, in practice however the results are rather disappointing. The most confusing part about this section is that it in large part looks like a Metal Gear'ish type of gameplay where you have to sneak past the space pirates without being noticed, mechanically the elements are even there, you got alarms that get triggered and spots for hiding. What destroys this section is that you are very frequently forced to trigger the alarm without any way around it. Thus instead of a clever game of hide&seak, it just boils down to running real fast to the next hiding spot to escape your followers. The gun you have is only good for stunning enemies, so you can't actually defeat them. What makes this section so frustrating is that you have no other choice as to painstakinly remember the path, as almost every unplaned enenmy encounter can and will result in your death. On hard mode the game even goes so far as to intentionally blocking a few savepoints in this section, making it even more frustrating.
In terms of length the game is very similar to Metroid Fusion, around four hours for the core game and then another few to collect every item. The game also includes next to the remake, the original NES version scaled down to GBA resolutions. The NES game automatically stores the password, so you don't have to write it down to save you state.
Overall I didn't enjoy this as much as Metroid Fusion, while the added freedom is a nice change, the game just lacks purpose and feels rather blunt. In Metroid Fusion each sector had a clear purpose and theme, while here it all just feels more random. It also just doesn't look as good and the suspenseful athmosphere and the intensity of the gameplay is mostly gone. The zero suit mission just feels like one huge missed oportunity and thus feels simply out of place.
In the end it is still a solid Metroid adventure, but my least favorite one of all the 2D ones, it feels to much like a step backward in some areas while not really offering anything new. As a remake of the original NES game it also fails, as it neither does a good job of capturing its atmosphere nor does it contain many recognizable parts and those that it does, where also remade for Super Metroid, so thats nothing new either.
Metroid Zero Mission for me marks the starting point where the series took the wrong turn. The first four parts build up a nice little storyline, where one part basically follows directly after the other, this remake of the first however doesn't add anything interesting of its own, but clobbers the story with some useless and unneeded pieces of Samus Arans backstory. It also introduced the zero suit and the overall new look of Samus, which seems to be a pointless depature from the more realstic and gritty lock before (as far as pixel art goes anyway).