Celeste is a short platformer about a girl climbing a mountain to overcome self-doubt.
Celeste lacks depth. Most screens have only one way to reach the destination. Other than a skippable platform or the ability to grab an edge or land right on top, there's usually a mandatory sequence of correct moves with very strict timing.
Compare this to Super Mario Bros. In Super Mario Bros, you can set your own pace throughout an entire level, most platforms are optional, and most threats have multiple ways you can deal with them. You can jump on enemies to eliminate them entirely, or just jump over them which is easier. Koopas can be used as weapons against other enemies and blocks. Powerups don't just refill your dash, they change the ways you can interact with the level.
In Celeste there are really only platforms - everything that looks like a powerup is fundamentally just a different kind of platform. (Consider this: how much would the game actually change if you could land on green gems? It would be easier because you could stop and think, but otherwise, it'd be exactly the same.)
Apart from the shallowness of its mechanics, though, Celeste gets everything right about the gameplay. There are quick retries, autosaves with reasonable section length, and I never had a single death that seemed like it couldn't have been avoided without foreknowledge. You always get enough time to react to things after you see them.
There is also an "assist mode", which is an excellent implementation of an easy mode. It lets you choose various advantages like adjusting game speed or unlimited wall-climbing stamina and you can turn them off at any time. I used it to get around a bug causing one section to be impractical. Uniquely, it's not framed as a persistent setting or something you're supposed to do, but as an emergency tool, encouraging everyone to only use it after seeing from experience that they can't proceed without.
The story was definitely more interesting to me than the game. I didn't much like the tedium of screen after screen of the same simple mechanics with sparse dialogue and no end in sight. (I was much happier with the chapter where you can see how many checkpoints you have left.) I didn't do the optional challenges and I definitely wouldn't have played the game if not for the story.
The creators really care about their side characters. Everyone you think is a one-time use NPC turns out to have multiple sides and be important and interesting in their own right. While the game is about Madeline's story, it really shows that everyone else has a story too, and that's awesome. The writing is really good. I don't think there was a single clumsy or disinteresting line of dialogue.
The artwork is really good, and the voice byte system is more sophisticated than Undertale's and allows a character's voice to change tone while still sounding like the same character.
However, Madeline's story has fundamental execution flaws that prevent me from rating Celeste more than "medium". Of course, I can't discuss this without spoiling it.
The prologue establishes Madeline's potential growth arc by briefly showing her as self-doubting or mildly depressed. It's foreshadowed that the mountain has magical properties, that it makes you face your true self or something, and the main antagonist is Madeline's evil alter that people seem to call Badeline.
Badeline originally identifies herself as "the pragmatic one" but here's where the story starts to fail: her actions don't align with that. Instead of just discouraging progress, Badeline is trying to actually kill Madeline (at least from our perspective and ludonarrative dissonance is never established).
Aside: after the first encounter with Badeline, it's revealed that Madeline dreampt it, but in later chapters the encounter is treated like it really happened.
Throughout most of the story, Badeline is framed as a metaphor for Madeline being sabotaged by self-doubt, but this is unsatisfying because we've never actually seen that happen. Madeline has expressed self-doubt and mild depression but we never see it hinder her, except when personified into an entity with its own powers and a separate will - which makes me feel that this is not a legitimate personification of part of Madeline.
When advising Madeline about confronting Badeline, Granny says "I've never seen someone so angry at themself". Anger is even farther from accurate - when did Madeline ever show anger at herself?
When Madeline defeats her, Badeline doesn't get vanquished or absorbed, but becomes an ally. This just doesn't work because it contradicts the idea that Badeline is not literally a separate person but a personification of a weakness. It would work if she became an ally in a way that still represented self-doubt (that is, if she became the voice of rational caution - but then that wouldn't fit this story), but the way it happens, she just ceases to represent self-doubt.
Pet peeve: when the two argue about forgiving Mr. Oshiro, Badeline has lines of the form "he did X", perfectly setting up Madeline to say "so did you", but she doesn't.
About Madeline being trans
This was claimed by developer Maddy Thorson, but sadly it's not in the base game (the author says it wasn't even her intention at the time), only in the DLC released a year later. I wish Madeline had been trans. That would've been really cool and fit really well into this story. But as it was decided only after the story was complete, it's like saying Dumbledore is gay. Maddy discusses that in the article, but fails to explain why this is different.
Of course, we underestimated how much anything less than a full, in-writing confirmation will be endlessly debated as "not enough proof," to which I ask, where’s the proof that she's cis?
Where's the proof that Dumbledore is straight? And yet, Maddy didn't like that one.
The answer is simply that most people are cis. Because of that, characters in stories are cis by default. If you want to make a trans character, there should be some indication of that in the original source material. It doesn't have to be completely explicit. The clues in the farewell DLC would've been enough; if they were in the original game, I wouldn't be complaining.
I'm afraid this will be taken the wrong way, so let me contextualize what I'm about to say. I'm a psychologist. I psychologize everyone. It's an exercise and a study. Suggesting motives for decisions I don't like is not an insult, a personal attack, or unusual. Now, from Maddy's writing, I think the reason this happened is because Madeline was a self-insert to her, so much so that after Maddy realized she was trans, she felt Madeline had to be too.