It's nice. That's almost it, really, although I liked its themes. It's not exciting, dramatic or anything else like that. There are no on-screen villains and every named character is friendly, helpful and eager to praise the protagonist.
That said, though, it's watchable and I enjoyed it. Importantly, it's not a wank fantasy about a mega-hero girl magnet who defeats enemies in his sleep. Ryouma is an 11-year-old boy who in his past life was a poor schlub in Japan who got worked to death by his employers. Now, he's been reincarnated as an extremely polite boy with lots of magic spells and a fascination with the weak fantasy monsters known as slimes. He can control them. He owns thousands and he trains them up to serve different functions. They look really useful, actually. I'd love to own some. He has cleaners, healers, scavengers, sticky slimes, acid slimes, poison slimes and more.
Everyone thinks Ryouma's amazing, especially for an 11-year-old. This is fair, since he is. What he's not, though, is Conan the Barbarian. When going into town to register with a guild, he starts out with "magical animal trainer" and only joins the adventurers' guild because calling himself "slime trainer" would make him the lowest of the low. (He'll join the merchants' guild too.) His adventuring jobs include cleaning a dirty house. Even when he joins a party to exterminate monsters in a mine, he doesn't actually do the real fighting himself. He just uses his slimes.
The most important thing in this show is Ryouma as a boss. Halfway through the series, he starts a laundry business. Does this sound boring? Well, yes, but you'd better get used to it. The show spends a lot of time on this and it's clear that this is its primary focus. His former employers were slave-drivers, so he's determined to be a perfect boss even in a fantasy world with slave-owners. He pays his employees well and gives them lodging, holidays and regular meals, for which he goes so far as to hire a professional chef. He's willing to hire people with questionable backgrounds and will protect their privacy in conversation with others. He provides healthcare personally, by using Healing Slimes to cure injuries. If things get busy, he'll always hire additional employees to ease the burden on the existing ones. He gives everyone every opportunity to voice complaints or suggestions. (There are absolutely none of the former, but some of the latter.)
(Mind you, I did notice that the show rigs things so that he never has to reject anyone. He never has two applicants for one vacancy.)
He's just as considerate towards his slimes, incidentally. Slimes are weak and in some cases can barely survive on their own in the wild. Ryouma treats them as his personal friends, protects them, helps them breed, feeds them their favourite foods and helps them evolve into new forms with powers they couldn't have developed without his help.
He has a love interest, Eliaria Jamil, but they're both 11. I'd be surprised if Ryouma had even reached puberty. (Eliaria certainly does like Ryouma a whole lot, though, and it's obvious what their relationship will be like in ten years' time. Incidentally, the gods who reincarnated Ryouma will tell him that his child body is reshaping his mind to be more juvenile, which feels like an author's saving throw to justify a budding romance between an ex-adult and an eleven-year-old girl.)
Adult characters will have a maternal or avuncular relationship with him. Everyone's nice. This world does contain bad people, e.g. the local government officials who cut workers' pay and pocketed the money, but we only ever hear about these people. They never come on-screen.
This is a gentle, mature show. It's quiet and definitely has that conflict-free isekai problem, but I quite enjoyed it. That said, though, you couldn't call it exciting and it's a much weaker show than, say, Ascendance of a Bookworm. (Both are pseudo-medieval isekai fantasies with child protagonists who go into business, but Myne's life is full of obstacles and Ryouma has none worth speaking of.) This is the kind of show where a storyline can start out with the town council having tried to cut costs, penny-pinched on waste disposal and created an problem with the town toilets. I quite liked it.