I don't see the appeal of I've been killing slimes for 300 years. From go it get's progressively boring and it's not necessarily due to the lack of interest in the genre itself, the anime is just lacking and doesn't have much to it.

The story is very episodic, with plotlines that last an episode or two, with continuity happening between event's about a woman in her late 20's who get's isekai'd to another world after working herself to death. Each plotline center's around something mundane though sometimes evolve into something less mundane like a dragon wedding which becomes a dragon brawl, though for the most part it's cute girls doing random things in a fantasy world.

Stories don't require action to be interesting, though there isn't much going for this one. It's an Isekai, but frankly the story would work just fine if it wasn't. Azusa, the reincarnee in this isekai, come's into the world mostly attachment free with the only thing that benefits the narrative of her being someone from another world is the explanation of her motivation for living the carefree life she desires. Outside of that she doesn't have any personal connection from her previous world nor has any reasons to want to go back. She doesn't have much in terms of personal baggage from her previous life nor is it mention what her skill set was in her previous life, so her being from another world is hardly something brought up as the story goes. There is much more which just snowballs into this big blob of undefinable wholesomeness .

-She's made immortal upon being reincarnated but outside of a 300 year time skip, it's not an aspect of her character that is utilized, and is more just to explain why she is over powered. She is more powerful then anything in the world currently in the story and often this power is really just used to resolve conflicts easily.

-It's a slow life anime but it doesn't do anything to really interest the viewer. It doesn't have the attention to detail for it's setting. Doing mundane things in a fantasy world would be interesting if those mundane things were made to be a bit more fantastical but maybe outside of one instance I can't really think of a moment the story leverages it's fantasy setting to show the viewer something fun or interesting, which goes into another issue that the story doesn't really focus on doing mundane activities despite being a slice of life. It feels like the story just has characters do things because it's something to do, rather an experience in of itself.

-Character's feel like they don't have much character, and are forgettable for the most part. Despite having 7 characters in the main cast, I couldn't really tell you their personalities except maybe the elf who is consistently ditzy, and a few of them have quirks though that's about it. They don't have any unique strengths divided among them nor can I recall any negative traits, none that maybe meaningful at least. This also makes the comedy not land. I really didn't know where to put this aside, as when there is comedy it doesn't work and it really has to do with cycling back to the characters not having character and lacking trait's, specifically negative ones to bounce of each other.

I suppose if your hungering for an anime that's just made up of wholesome cute girls moments then I've been killing slimes for 300 years might be something your looking for, though if not I suggest skipping this title.