There are lots of issues with it, but if I was to single out my biggest gripes, they'd be the OverPowered protagonist done wrong and protagonist himself as a character. Other less major issues to note are bad worldbuilding, predictable plot and inconsequential supporting cast. These matters are honestly very simple and others have written much of what I think in the most helpful reviews so I'll focus on my major gripes.
So, how do you do OP protagonist wrong? Well, him blasting through every challenge with a spring in his step is one sure way to do it. By the halfway point of the anime they could announce that Rimuru is about to face god and you would assume god is about to get dunked on hard judging by how things have gone so far. He faces enemies of mythical scale with his posse and suffers no casualties or severe damage. And worst of all, he is still growing in power with no hints of there being limits to it. There are less stakes here than in your average Saturday morning childrens cartoon. A good way of doing OP protagonist would be to have him face at least one enemy of equal or above level or give some kind of limit to the protagonist power growth or have him be OP from start but with knowledge that there are still credible threats to him.
Then there is is his personality. I was really hyped after the first episode, when he declared he is going to bang every woman he meets if he gets the chance, after regretting living to near 40 while being a virgin. I thought, "Finally! Isekai protagonist that will go full hedonist when given the chance at new life with great powers!". Tensei Shitara took my hopes&Hype and proceeded to punch me in the gut with them. When he gets human form, does he turn himself into a big hunk of a man and do every woman in sight? Nah, he sticks to androgynous loli form and never even kisses one of countless 10/10 beauties that cling to him. Just reiterate, this is the same guy who died vowing he'd lose his virginity the first chance he gets if he gets another chance at life. This, topped off with his massive messiah complex, that the world around him accommodates fully, makes it really hard to root for him. I was prepared for Tensei to disappoint me, but I didn't think I'd get bait and switched by it.
