Isekai is a genre with great potential.
That potential usually goes unfulfilled.
300 years a slime murder fulfills said potential.

A staple of isekai is an OP protagonist, but what's holding most series back is that they fail to understand the appeal of it. "Our protagonist is OP, so the villain should be OP too" is the standard school of thought. Having an OP protagonist is the primary prerequisite of an action/slice of life story, because otherwise the action takes up too much space in the story and the protagonist being in real danger makes it hard to believe that it's someone's life. Add an OP villain and the only thing affected by having an OP protagonist is that the world is forced to be at stake (which is generally accepted as bad storytelling) and to give the girls in the harem something to swoon over.

300 years a slime murder has 6 fights up to where I've read. 2 end before they even start, 3 are over in one hit and the last isn't very long. This is perfect if you're like me and despise watching fights. This adds up to one roughly half-chapter long fight every 12 chapters. A series I recently dropped "Tensei Kyuuketsuki-san" bothered me because in the first 15 chapters it had like 10 fights.

But of course, "not being an action series" is not good enough reason to praise a series. Most series aren't action series.

- The series encourages the good practice of not overworking oneself.
- "And then 300 years passed", reading that is an interesting experience.
- In general, I find immortals interesting. It helps me put my life in perspective to contemplate the mindset of someone 300 years old.
- The slime loli's are pretty cool.

The most important thing though, is that this is an isekai series that doesn't make you sit through long and boring fights. Isekai and action being intrinsically tied together is tremendously infuriating, to see them separated in a series which passes basic competency checks is enough.