The first season of Demon Slayer is rough. Really rough. It has a few decent themes to it but they all fall down to surface level ideas. Such as the theme of family. Or its lazily written plot progression. Or the fact that every character follows either a stereotypical Shonen trope to a T, or is just incredibly insufferable. Or the very shotty CGI animation in every scene that is not only noticeable but takes you out of any immersion you had where they try to make it look natural.
The characters of Demon Slayer are all shown as quite annoying. Zenitsu is the biggest pile of dirt I've ever had to watch in anime, he's arguably worse than Meliodas. There's nothing redeeming about him, every time he gets a moment to shine, like protecting Tanjiro's box. That was a good moment to show his respect and actual appreciation for Tanjiro despite being in harm's way. Unfortunately this one character moment they give him was completely undermined for the sake of humor. Once he finds out what's in the box he goes ballistic and then focuses back on himself like the horrible human being he is. This trait is then used as his primary joke where his whole deal is that he wants to be "friends with benefits" with Tanjiro's sister. Tanjiro himself also suffers from lazy main character syndrome where he not only doesn't know things that make zero sense that he wouldn't but also he seems to lack the ability to explain any misunderstanding to create either forced comedy or forced tension. It's incredibly frustrating and can easily be rectified. For example, when Tanjiro is being accused of something, he easily can say "Let me prove it!" As he already knows that the situation will turn into his favor. However, he sits there and just says "No! No I won't let you" blah blah blah other generic stuff.

Same goes for the misunderstanding of his sister being in the box. Zenitsu thinks it's romantic but Tanjiro is too dense to literally just say "she's my sister." And even if he were to say this off-screen, it's weird that Zenitsu still wants to be in a very touchy relationship with his sister. Inosuke is the most bearable of the bunch but he still has his issues. Most of the jokes don't land but he can be enjoyable in some areas, especially in the latter half of his appearance. He actually has a good dynamic with the others and he works as a supporting character.
Main characters out of the way (because every other character isn't even worth mentioning in this season), the plot is very surface level. Revenge/avenge and also cure. It's also very predictable with how a scene will play out. The twists present with villains can be seen a mile away other than one weird fake out. The way the story progresses is also the laziest way of making things happen, "go here. Now go here, and now here." There is no actual natural way the story progresses, Tanjiro is simply told to go one place after the other by his bird which makes the story not only uninteresting but proves that our characters have nothing to do with the actual plot other than being at said place to beat up bad guys and move on.
The world is just the Taisho Period of Japan, it just has demons. The fact that this series has won best fantasy compared to literally anything else is not only a travesty but incredibly stupid. Fantasy elements and full blown fantasy are quite different. Another nitpick I had was continuity errors. Inosuke pointing in the wrong direction of where a demon is located around episode 16.
Another nitpick is how like I said before, Tanjiro doesn't know what a Hashira is despite being trained for 2 years straight and having met one already. Or how they show him meeting the main villain of the story for the sake of the meeting and nothing else. Or how in the last climactic battle they have like 3 different level ups happen which feels quite forced in terms of hyping up the scene. It was still fun though and definitely leads into my favorite part of the season.
Somehow I enjoyed the last 4 episodes a lot. It was a bit bad in terms of pacing since it was a mini training arc but it was enjoyable just seeing the characters interact with very little forced comedy, especially Inosuke. It was enjoyable and I had the least amount of issues with that part besides the pacing. Everything else was great.
At the end of the day though, Demon Slayer's first season is the definition of mid. Nothing too exciting or even deep beyond a surface level with lazy writing and annoying characters that end up making it whatever. If anything, I find it generous to give Demon Slayer this rating, as you could argue that it not only fails at being anything surface level but fails in so many other departments.