It's an isekai show, i.e. someone from our world gets transported into a fantasy one, where he has game-breakingly amazing powers, attracts all the girls and is basically just super-mega-awesome just because he turned up. (That's an unnecessarily detailed definition of the sub-genre, but at least some of it is usually true. The exceptions are often deconstructions and/or parodies.)
In this case, it's pretty much on the nose. Ichirou/Satou is all those things. He was a game programmer in our world, but now he's a Level 310 Everything, with an ever-growing list of titles, skills, inventory items and harem members. He can basically do anything, especially if you give him time to prepare and practise. He also has very little characterisation, as does most of his harem. I've ditched other shows for doing all this, e.g. In Another World With My Smartphone, so why did I watch this one? (I even quite liked it.)
The answer, I think, is that the show (to me) doesn't feel like a hand job. It's not primarily a power fantasy for people who also want worrying sexual undertones, although that's definitely in the mix. It's exploring its world. I didn't get the impression that the show existed to show off Satou's awesomeness, but instead to dig into this fantasy society and its game-like rules, poke around puzzle points and build up a sort of family. Satou's power levels are ridiculous, yes, but that's almost always irrelevant because there's almost no real danger here and it's usually more of a slice-of-life show. Instead of just doing everything himself (which he could easily have done), he gives his companions jobs to build up their XP and skills. He relies on them when they're more knowledgeable than him about dungeoneering. He eats fried bat wings, which look fairly gross. He doesn't try to overthrow this pseudo-medieval society, but instead does mundane stuff like trawling for accommodation where his semi-human companions won't draw racist abuse. He goes grocery shopping, learns how to cook and haggles with estate agents.
I really enjoyed all that... but I appear to be in a minority. Lots of people found it boring as hell. It's not trying to be exciting, fight-filled, sleazy or anything crowd-pleasing like that. Personally, though, I found myself liking the show a good deal and ploughing through its episodes at quite a pace.
What's different about this show and its world? Well, there are some things:
1. SLAVERY. This world has slavery and our hero ends up owning a bunch of... okay, yes, they're all girls. The demi-human races seem to get sold much more often than humans, but anyone can become property for something as minor as overstaying your visa. Satou's attitude to this is fairly neutral. He doesn't get outraged and start delivering speeches or anything. He just accepts that slavery exists and gets on with his life, but that won't stop him from being nice to people. However if you're being nice to slaves in a slave-owning culture, you're soon going to find yourself acquiring some followers who actively want to be your property. (They'd see it as either you or someone else, after all.)
Should Satou have given all his girls their freedom? Maybe. Sell them to themselves for a token sum. That might have raised complications in a world that may or may not have been familiar with the concept, but personally I think that if he wasn't going to do that, he should definitely have written a will. Dungeoneering is dangerous. It's irresponsible not to be prepared for the worst.
2. DARKNESS. Admittedly the show's tone is relaxed and light, but this is still a racist, slave-owning world where priests will sell rocks to stone people to death. There are child sex slaves. (Satou acquires two of them, so everyone has a big surprise that night. Since they're only aged 11 and 14, Satou tells them to stop it.) You can be a princess one day, then sold into slavery the next as your entire family gets beheaded.
3. GAME MECHANICS. This is quite detailed. Satou spends a fair amount of time exploring different methods of extra-dimensional magical storage, for instance, or learning how to make a potion and a vessel that won't immediately destroy its value. He does little experiments. He makes mistakes. He works out how the magic system works and how to combine two rubbish spells to make a third one. This is also all clearly based on computer game mechanics, which interests Satou on a professional level.
4. THE REAL WORLD. The first episode spends quite a long time on Satou's real world job and the murderous hours he's putting in. That's the "Death March". Compared with that, fighting dragons is almost relaxing. Oh, and he's 29, which is pretty old for this kind of anime. (He's reincarnated into a 15-year-old body when he arrives in the fantasy world, though.)
5. SEX. The show's pandering to fans of, uh, girls of a certain age. I can't deny that. Satou's girls are mostly young and there are a few suggestive scenes. One of them (Arisa) keeps saying she wants to shag him, despite being only eleven. (Technically she's a reincarnated Japanese adult like Satou and indeed older than him, but... yeah, her body's eleven.) There's a childlike multi-bodied dryad who can only recharge magic points by kissing, while the last episode goes into the gutter a bit with erotic licking, discreet-ish toplessness and so on. That said, though, Satou never takes advantage, partly because it would be pretty disturbing as he's the girls' guardian/owner. On top of that, though, he's not even interested in shagging his young charges. He prefers older women, c.f. the scene in ep.2 where he's being shown around by an implausibly buxom 13-year-old... and he's more interested in the girl's fat mother.
Surprisingly he does even have sex a couple of times in this series, but only with prostitutes and waitresses. (That's unusual for a harem show. Every time he does it, Arisa complains.)
6. LOTS OF THE GIRLS ARE VERY YOUNG. This makes one or two scenes a lot more problematic (although it's just the show being silly and nothing actually happens). For the most part, though, this defuses the harem angle and stops the show from feeling sleazy. Satou's almost in a parental role.
7. IT'S NICE. Okay, that's fairly normal for anime. Harem shows in particular are often heartwarmingly nice, believe it or not. However it's still true here and definitely a factor in what made the show enjoyable for me. Satou doesn't treat his slaves as slaves, instead ensuring that they have decent food, clothes, shelter, weapons and training. He notices if they seem unhappy and tries to do something about it. He makes dolls for them. (No, really. He makes one in ep.10 and suddenly everyone wants one.) I wasn't happy when Satou got dragged into a dungeon on his own in ep.8, because it was cutting off the show from all the interaction (with world, culture and characters) that I'd been watching for. (The episode's okay, but I was happy when the gang got together again.)
It's not a perfect show. Occasionally I noticed cheap animation (especially in ep.3) and I don't think the tragic villain in ep.9 is completely successful. He's okay and I like the callback to the stage play in ep.6, but I wasn't crying for him or anything. Also, more fundamentally, this is an understated slice-of-life show that will frustrate anyone who was hoping for sword-and-sorcery action, character-based drama or even just plain sleaze. Nothing can beat Satou in a fight. It's possible to slow him down and give him problems, but this is a guy who can max out any skill at will, can invent new magic and carries an energy-blast gun in a fantasy RPG setting. I'm surprised that I found the show as watchable as I did, but the secret of course is that Satou has better things to do than have fights.
I enjoyed it and I hope they make a second season. I can see lots of reasons why it wasn't universally popular, e.g. people having problems with a story that features slavery and doesn't have its hero single-handedly abolishing it. In a low-key way, I found it charming.