This game has some incredible level and boss design. The bosses are not hard, but you usually need to figure them out, and it's relatively easy to do so and you don't need to spend long times grinding the bosses. They hit this awesome balance where you don't spend too long with bosses, but it's not a complete cakewalk either. The bosses are also designed so that when you learn their phases, you can get through the individual phases extremely fast, so you don't have to grind through 2 stages over and over to get to the third stage. I also love how they use Samus's mobility in some of these fights to great effect. After playing through the annoying grind bosses of Kena, it's so damn great and refreshing to see bosses done right.
Every room is also so well done, with different challenges and different ways to engage the player constantly, there are so many smart subtle ways the game guides the player, you can run through 80% of the game by intuition. But they also have a few sections where you have to shoot some walls and think about where to go, to change things up, but those sections are also perfectly tuned to not become long and boring, you always find the next area before it turns into frustration.
The game has an absolutely absurdly well-done progression and flow to it, it leads you onward from place to place and room to room in an elegant addicting way, this is really where the triple-A 60 dollar nature comes in, it beats out Hollow Knight and Ori in just how well it progresses from hour to hour and what it offers the player in each hour. Even though I'm not gonna say it's better than those games overall, it's in the same league.
I also love how it ties into the other Metroid games later on. But I'm not gonna spoil anything.