It's a warm, generous story about heroes who are nicer than the fantasy world they're in. They don't even kill villains who'd just summoned an uncontrollable magical demon capable of destroying the world. Ultimately, at the end of ep.12, it's about kindness and forgiving people.
That said, though, it's also a harem-based fanservice anime with lots of bouncing boobs. It has a ridiculously overpowered chaste hero who can defeat all opponents (usually in seconds) and keeps being aghast at finding himself in sleazy situations. Oh, and there's also slavery, because that's become an occasional thing in anime in the last few years. Heroes go to sword-and-sorcery worlds and become the legal owners of other human beings! This is, supposedly, cute and harmless. There's even a happy, heartwarming slavery ending.
I liked the show a lot, but it's not for a general audience.
It's quite an interesting show to discuss. It has stuff that's legitimately cool for reasons that you'd defend to intelligent people. It also has silly sleaze that would have you lunging for the "off" button if someone walked into the room unexpectedly, but even there I sensed (a very small amount of) restraint. I have a feeling that the anime producers were pulling back a bit from and deepening the source material, although I can't substantiate that since I haven't read either the light novels or the manga. Nonetheless, though, I believe that only in the anime did Diablo not kill that Fallen in ep.5 (although what he does instead isn't exactly merciful). The anime also explores Alicia's psychology far more thoroughly, among other things.
Then there's the fanservice, which is inexplicably going for modesty. This is a pretty weird decision for a show that's full of ridiculous cleavage and all kinds of inappropriate scenes. Nonetheless there it is. Ep.1 has Light Beam Censorship, but what's on the uncensored Blu-rays is hardly any different. Still no nipples. The fabric-eating slime in ep.7 is family-friendly. The girls can get naked in a river and you still won't see anything. This feels to me like a show that's cheerfully adapting everything in the source material, no matter how outrageous, but in a sleight-of-hand way that's steering away from top shelf material.
I could imagine giving this show to female fans. They'd need a sense of humour, but there's plenty of anime out there that's genuinely unhealthy and/or uncomfortable to watch. This show isn't that at all and indeed I understand it was one of the hits of its season.
Its secret weapon is ex-hikikomori Diablo, who's far more memorable than most anime protagonists. (That statement applies to anime in general, not just its particularly egregious sub-genres of harem and isekai power fantasy.) Instead of just being another gamer nerds who's found himself in a gameworld, our hero is the poster boy for pathetic loser virgins. He has two voices: (a) his swaggeringly macho evil outer voice as Demon Lord Diablo, and (b) his terrified, easily intimidated real self inside. He's so timid and rubbish with people that he can't speak at all unless he's play-acting as Diablo, which puts him in a quandry if, say, he needs to be polite. "I managed a peaceful introduction with my poor communication skills!!!!" Inappropriate contact with females will give him a cold sweat. His horrified reactions can turn a fanservice sleaze scene into comedy. Diablo's inner thoughts never stop being funny, partly thanks to Masaaki Mizunaka's voice work. (At his silliest, he becomes cute.) I loved him. He makes the show twice as watchable.
He can even be useless and self-pitying in a way that feels real. When short of magic points, he'll retreat to bed and just lie there like a slug. (He's reverting to type!) Similarly he can be sent into a spiral of self-doubt by one of his friends appearing to reject him, even though he's genre-savvy enough to have guessed the real situation. (He doesn't know for sure.)
I also liked the supporting cast. Shera L. Greenwood is of course a walking pair of boobs, but she's also such a kind person that she'd even protect her brother. (This brother had brainwashed her, started a war and tried to destroy the world out of pique. Oh, and he'd also been going to make her have his children.) She hated him, but she'll still grieve over him. Rem Galleu I liked too, but I wasn't entirely convinced by her portrayal. She gets a strong role in the season's last story arc, but even so I don't think the show ever really made us feel her pain. She has life-changing issues. She's never known anything but fear and isolation. In practice, though, she usually just seems haughty. This isn't bad enough to break the show at all, mind you. The character's fine. I don't have a problem with her presence. However the show's usual tone is warm and funny, while I think you'd have to go darker and harder to portray Rem properly.
Similarly the show's portraying its world and its villains a little lighter than they could have been. Sex slavery is socially acceptable and you can have a pleasant chat with the slave trader. (Admittedly the slaves all seem happy and well cared for and we can't prove that anything improper's going on... but seriously. Look at them. Those girls don't seem to have been chosen for physical strength, ability to perform manual labour or anything like that. Instead they're attractive and wearing nice dresses.)
In the same way, Saddler's less horrifying than you'd expect of a man of God who's into torture and massacring entire communities. He's villainous and we want him dead, sure, but we're not scared of him.
(On the slavery thing, by the way, Japan has history there. It had its own official slave system between the 3rd and 16th centuries, while Japanese people themselves got sold into slavery abroad by the Portuguese. Sometimes they'd even become the slaves of slaves, with Malay and African slaves themselves having Japanese slaves of their own. Large numbers of Japanese slave girls also got sent to Portugal for sexual purposes.)
I like the fantasy world. Diablo knows the original game inside-out and makes the show smarter by identifying genre tropes and plot devices. There are interesting touches like the people of this world being lower-level than Diablo's used to, because here death means death. You can't just reboot for another go. This world's people aren't level-grinding, but instead living their lives here.
I genuinely rate this show. I responded to our heroes trying to be nice in a world that's not. They can only get away with it because Diablo's level 150, but he is, so they can. He's so strong in this world that in ep.5 he buys a weaker weapon so he can fight opponents without being scared of killing them. Besides, the show's soon looking for proper opposition for him. There are humans in this world who could threaten him, even without getting into the monsters and demon lords like the Fallen and Krebskulm. (I loved where the Krebskulm story ended up going.) Even the slavery thing isn't a problem, since Diablo completely ignores it and never gives his "slaves" any orders, instead just treating them as friends. They themselves correct a hero who tries to "rescue" them from bondage. I hope they make a Season 2, even if this unfortunately gets more harem-y with more girls on top of the core trio of Diablo, Shera and Rem. (NOTE TO SELF: fix unfortunate wording.)
At the same time, though, it's also shamelessly trashy. Diablo's personality defuses a lot of the cringe factor (e.g. getting a sweat drop of embarrassment when he says something particularly macho), but it's still ludicrous. Girls fawn on our hero. We learn the Big Boobs Method of Potion-Making. You wouldn't believe what can help you get back your magic points... okay, no, maybe you would.
It's still fun, though. I charged through it at a rate of knots and I'd have watched a second season if I had one.