Frankly, That Time I Got Reincarnated as a Slime succeeds without any problem in what it was meant to do: entertaining the viewer. It has funny moments, it has well-written characters that almost never show way too exaggerated traits or tropes that make you hate them, it has a whole journey of evolving, not only as an individual creature (our protagonist, the slime), but as a whole of different species as well.
So, let's go over the "flaws" of this show.
1. The direction-less plot
While I see why you could think that, I don't necessarily agree. The plot is a little blurry, yes, but that's mainly because the MC itself is pretty clueless about the world and doesn't have a clear ideal in mind, so he just goes with the flow, helping the characters he likes and expanding his town. Which in all honestly I think it's more realistic than expecting someone thrown randomly in a completely foreign world to have a clear path and objective (also, half-way his goal is mostly to complete Shizu's wishes).
2. The MC is too OP
He was actually defeated by one of the Demon Lords, Milim. And would probably have been by many others, so I don't see the point of this. He also wasn't this powerful from the start, he gained his skills over time, mostly thanks to his Predator skill.
3. All of the bad guys/enemies defeated stick to him
Honestly I have thought about this at some point too, but most of the time their actions made sense. Most of the time. Because there were also the Dwarf Brothers and Phobio (though the latter didn't exactly join the party, just apologized). Aside of that, I think it makes sense. The goblins were intimidated by his aura and asked him for help, and he did save them, so obviously they were loyal to him. Also, wolves usually follow the strongest, and since Direwolves are also wolves (kind of) I think it made sense for them to follow him. The ogres situation was also very realistic, since at the first there was only a alliance to defeat the Orc Lord, they decided to acknowledge Rimuru as their Master only after they saw his powers and how life in his town is. And the list goes on, but I think they handled this pretty good (with a few exceptions obviously).
4. Harem
There are some traces of harem, indeed, but I don't think they spoil the quality of the show. Girls usually think he's "cute" because he's a slime, and after he shows his powers/leadership because he is strong. But I don't think that it's so unbearable that you can't overlook it, if you want to. Fortunately, not every girl is head over heels for the MC, and the harem itself isn't shown that much through the anime, there are just some moments that hint at it. And yes, a lot of them have big boobs that are carelessly flashed on the screen even in battles and shit, but honestly what's the big deal about it bro? Just some boobs.
I have to admit tho, that Chloe girl falling in love (or whatever was that) with the MC didn't feel quite right. I'm sorry storyboard directors but what the fuck was that? It was neither necessary or appealing, just so you know. It was rather uncomfortable.
Well, anyway, overall I think it was pretty good. Nothing too amazing and by no means the best anime I have ever seen (it doesn't even get close), but not quite bad either. I really don't understand the hate on it, but after all people have different opinions and views on any subjects, so in the end it's just natural. But if you like isekai that's not just full-on packed with only action, but contains a bit of slice of life too, I think this might be a good match for you!
