"Two steps forward, one step back" is how I would describe Metroid II: Return of Samus. for every genuine improvement made to the original game, and there are a lot, there's an unfortunate compromise made due to the inherent limitations of the GameBoy.

let's start with the improvements. for one, Metroid II is a lot less frustrating than its predecessor for multiple reasons. firstly, Samus is a lot less slipery, and so she no longer slips after landing. controlling her is snappy and responsive, and the floaty jump means it's very easy to land where you want to. the newly added abilities to crouch before entering the morph ball as well as shooting downwards are a god send and singlehandedly make the controls a lot smoother.
secondly, the level design is a lot less dickish. there's no massive dead ends, platforms are wider so they're easier to land on, no beginner's trap, as well as a lot more landmarks and unique rooms, meaning navigation is easier and you're less likely to get truly lost. Metroid II, unlike the first game, never purposefully wastes your time, which is greatly appreciated. this, coupled with the addition of save stations and energy/missile recharges, while few and far between and the latter being hidden, means the game overall is way easier.
additionally, enemies deal less damage, and due to the capacity of the GameBoy, creatures can't fill the screen without setting the poor console on fire, so there's less enemies and you're never overwhelmed. this allows creature design to also be more detailed ironically enough, as sprites are generally a lot bigger due to the reduced screen size. this means there's no slowdowns which is also appreciated, the game runs at a crisp 59.7 FPS and never dropped once. the only true challenge comes from the titular Metroids who can be pretty difficult to fight especially when they surprise you, though they're never unfair. I also appreciate the added enemy variety. the wildlife of SR388 all look and act distinct from one and another, a lucky side effect of the GameBoy not having color.
while in the first game, Samus started as weak and grew powerful, here Samus starts powerful and eventually becomes unstoppable thanks to the reduced damage and new abilities like the space jump, spider ball and spring ball which all make exploration a breeze. there's also the fact that enemies outside of Metroids never really attack you, they just kind of exist and accidentally are in your way, which I really like as a story detail: the only hostile entities on SR388 are the Metroids and Samus herself. the rest are merely living. smarter people have already analyzed the story which you can read if you're interested in, but I like how Samus rescues the one baby Metroid. it humanizes her in an unexpected way.

now, there are a few elements that are transferred over from the first Metroid, mainly the lack of a map, which while not as detrimental this time around due to the improved level design, still means it's easy to get lost. the final boss is also a frustrating damage sponge and an unnecessary difficulty spike, tho it doesn't throw a billion projectiles at you which is a definitive upgrade. you also can't have more than one beam type, meaning I was just using the ice beam for 95% of the game and renders all other beams functionally useless, which is a shame. there's also more energy tanks and beam stations that you can collect which is just a strange move that I don't fully understand.

now for the downgrades. notably, I don't think the screen crunch is too much of a problem here, since the game designers were smart and built the game around it. you're never unfairly attacked by enemies you couldn't see coming, and it's not disorienting. it replicates the feeling of exploring a dark, uncharted planet. I would've liked a bit more screen space but it's not the end of the world.
no, my biggest problem with Metroid II is that for 90% of the game, there's no music, and I don't know why! the music that is present is very good, and I know they could've made oppressing ambiant tracks because the original game had songs like that. here however, you're accompanied by silence most of the time which is a shame.
there's also no color at all, which just makes the environment feel very repetitive and more confusing than necessary. the game is also shorter and more linear, which means you don't really get to use your abilities to their full extent, rendering them kind of superfluous which is never something you want. the game is also very repetitive, since it follows the exact same formula with no real surprise.

overall though, I have to say I was pleasantly surprised by Metroid II! I consider it an improvement over the first game, with most of its flaws coming from the inherent limitations of it being a GameBoy game. this makes me very excited to see what Super Metroid is capable of, especially considering everything I've heard about it.