I have a weird history with the Metroid series in that most of my life I had no history with it. In spite of the fact I've loved nearly every non-linear platformer in the Castlevania series over the years, Metroid had somehow eluded me. One day I saw a cheap copy of the original Metroid for NES in a used game store, and I decided to give the series a try. In spite of some of the game's general oldness, repetitively designed environments, and password-based save system, I really got into. I liked it enough that I actually beat it, making it one of the very few NES games I've ever completed. I went on play through Super Metroid, Metroid Fusion, and Metroid Zero Mission, in that order. I consulted my Metroid loving friends, and they almost unanimously told me not to bother with Metroid II: Return of Samus, and the only reason I ended up purchasing it was because of the low price.
After playing a good deal of it, I can see how people would have problems with it, but I also think they adequately explain its charms. Maybe I just shouldn't have weighted my friends opinions as heavily as I did, because most of them also told me to skip out on the NES original as well, and I mostly enjoyed that. Metroid 2 is a lot more of a sequel than I would have guessed. Instead of being a pared down version of the original Metroid, it actually expanded upon it in some useful ways. The controls are improved, allowing for crouch shooting, shooting downward during jumps, and morph ball jumping. The movement and attack controls are just as solid and responsive, but there are more options without things being overly complicated.
The level design is a mixed bag. One way it improved on the original game was something that I think was a consequence of the Game Boy's limited capabilities. In the NES game there were a lot of repeated room designs that only varied in color, and sometimes not even in that, but with the Game Boy's four colors of green (or later gray) the level designers were forced to me more creative in structure. There are a lot more large rooms, and most importantly, they aren't all made up of stock horizontal and stock vertical rooms. The level design is not perfect however. The different sections aren't all that different in the way of components, so you'll see a lot of the same rocks. One thing I do enjoy is the save system, which actually saves instead of using those horrific passwords, and incorporates save points throughout the map.
Metroid 2's most unique feature, other than the spiderball that allows you to climb walls and ceilings, is the overall level structure. It is more linear than most Metroid games, but it in no way discourages exploration. There are still a lot of hidden nooks and crannies, but instead of going back and forth between the differently designed areas, you work your way through relatively small subsections and there is never a reason to return to them unless you missed something. It's a weird design choice, but for the most part it works. Even weirder, is the fact the game's map doesn't actually make sense geometrically. Seriously, go look at a map and you'll see that there are lines connecting a bunch of points that wouldn't actually fit together. It is one of the reasons I couldn't imagine playing through the game without a reference map, even if it kills the mystery.
The story is not something that Metroid 2 dwells on, in fact it doesn't overtly tell it's story by way of text or anything like that, but when you actually think about what Samus is doing, it is kind of fucked up. The events of the first game proved that metroids were a threat that needed to be kept from the hands of space pirates, so in this game Samus has been sent to the home planet of the metroids to kill all of them. The main method of progress is genocide, as you have to completely clear each subsection of metroids before moving on to the next area. The game does acknowledge how fucked up this is by having Samus be unwilling to kill the final metroid, that happens to be an adorable baby, though it hardly does the subject matter justice. Even if it is the black sheep of the Nintendo R&D1 developed Metroid family, even more so than Fusion, it is still a decent game and is pretty impressive for something on the original Game Boy.