Overall
I recently heard someone apply the clever term "Metroid-brainia" to this game. Certain areas don't seem readily accessible from the beginning, but as you explore, you'll gain what you need to unlock them. Where in a typical Metroidvania, you'd gain items or abilities needed to access those areas, here it's knowledge you need to gain, and once you learn how to reach an area, you can get there with nothing more than the basic items and abilities you start the game with.
Where Outer Wilds really stands alone even among other games with knowledge-based progression is that it's entirely self-guided. The game never gives you a single objective to complete - not even a grand overarching one to tell you what your ultimate goal is or how you'll know when you've beaten the game. It simply throws you into a world where something is happening and trusts that you'll be intrigued enough to try to uncover more information; that as you learn more, you'll begin to see that there's a role for you to play; and that as you learn even more, you'll eventually figure out what it is.
All of the above means that the game is more rewarding when played totally blind. (It also means the game doesn't have much replay value, but I wouldn't count that against it.)
Story
The emotional impact of the mystery and narrative can't be overstated. The core message is one that I'll probably spend the rest of my life thinking and having feelings about, but even the way that message revealed itself to me carried a lot of emotional weight.
The nature of your investigation is that you'll most likely form a few initial hypotheses and modify or discard them as you gather more information and a clearer picture starts to emerge. There were points at which I suddenly realized that no, something I had been assuming all along was happening here is actually something quite different, and that means a bunch of other stuff I thought I understood actually means something much different than I first thought, which often felt like a galaxy brain moment or a gut punch (or both).
It makes you realize how rarely we truly have the experience of being wrong and being forced to revise your understanding based on new information that you didn't have when you formed your initial opinion. In real life, we tend to overindex on evidence that confirms our initial opinions, ignore evidence that doesn't fit, and rarely change our minds about much. When we do, it's usually because someone else made a strong persuasive case. This game forces you to be intellectually honest in a way that real life rarely does, and that in itself is a worthwhile experience.
Music and Graphics
Soundtrack is understated, with a lot of quiet tunes and even some stretches of silence, but this is highly effective at setting the mood and calling your attention to ambient/environmental soundscapes. Certain recurring riffs continually pop into my head even days after last playing the game and give me another welcome opportunity to think about the game and its message.
Graphics are low-fi with a charming cartoon art style. The game uses a first-person perspective, which I virtually never play and frequently have trouble with due to motion sickness, but luckily this game didn't trigger any malaise. I suspect this has to do with the compact size of the planets and the vast emptiness of the space environment - neither has to employ depth of field/focus effects (which are often responsible for motion sickness among folks susceptible to it) to try to realistically render a detailed but distant background.
Controls
A title card when you boot the game recommends using a gamepad, which I did.
Movement might have a steep learning curve - the controls are a little bit soupy, but they can start off seeming a lot worse than they are because they're based on semi-realistic physics in zero/low-gravity frictionless environments, so momentum plays a much bigger role than players will be used to compensating for in platformers with normal gravity and friction.
There are a couple of points where this game will randomly skill-check you with a difficult platforming section and these did feel slightly uncalled-for in a game that's primarily about exploration and puzzle-solving. These are the main reason I gave it 4 stars instead of 5, because struggling through two such parts were the points at which I came closest to quitting the game because I began to think I was never going to be able to clear them.
In each case I took a couple of days' break and ultimately came back because I was so motivated to solve the game's mysteries that I was willing to try again. In one case, I needed to look up a Youtube video so I could just see what successfully navigating that stretch was even supposed to look like and adjust my own strategy accordingly.
Puzzles
Puzzles are all essentially finding the answer to the question, "How do I get to [location that I have come to believe is important to my investigation]?" Most of the time, the trick to getting past or around an apparent barrier is eventually made obvious in one of two ways: you find a clue that spells it out very clearly (but only after first following a trail of other clues that led you to the information you needed), or you planted yourself in one location long enough to be in the right place at the right time to observe the solution being shown to you via time-dependent events.
There are a couple of places where the signposting could have been a bit more clear. On two occasions, I ended up looking up answers after being stumped for a long time and having no other leads to follow up, and discovered that I had already come up with and attempted the correct solution, but I didn't execute it in quite the right way, and when it didn't work I mistakenly took that as indicating it wasn't the right solution.
About hints and spoilers...
The places where I mentioned looking up a YT video or solutions to puzzles I was stuck on did result in stumbling over a couple of spoilers that I didn't want to see. Luckily, none of them were major.
I wish I had visited the game's subreddit early - I had avoided it because I assumed it would be full of spoilers, but it turns out folk there are not only really great about hiding spoilers behind the appropriate tags, as a community they're also really great at giving hints without spoilers - things like "Go back to X area and look around again, maybe just hang out for a while and observe what happens," or, "Was there something you were looking for in Y area? Do you have any tools you could use to help narrow down where it might be?" Or even, "You've had the right idea already, but your execution was off. Try again, but see if there's a different way you could go about it."
If I had known, I would have been a lot better off asking for help there when I got stuck instead of trying to avoid spoilers on Google. They would have been able to direct my attention in the right general area to be looking without robbing me of the chance to work out the meaning of what I was seeing for myself.