It's pretty dark, both in its plot and its protagonist. Subaru Natsuki is the nicest guy in the show and he'll go through hell to help people, especially Emilia... but he's also a brutal deconstruction of such heroes. He thinks he can do anything because he's the protagonist, despite having never achieved anything before. He's childish. He can be manic, arrogant or overconfident. He can also be pathetic, annoying, a despair-hollowed zombie or an ugly, ranting egotist. The only characters weaker than him are children. He gets killed repeatedly. (Yes, you read that right.) At one point he'll give up and try to run away. He's a loser who'll fail, fail and then fail so hard that it hurts before eventually succeeding. He's earned in blood the right to most of his reactions, but he's capable of toppling into self-hatred and screaming everything on his not very stable mind. This can be uncomfortable to watch.
This show's riffing off the same genre as KonoSuba: God's Blessing on this Wonderful World!. Both have a nerd hero protagonist who's been transported by a supernatural being from modern Japan to a fantasy world. Both shows are genre subversions with deeply flawed loser protagonists. Both Subaru and Kazuma can return from the dead. KonoSuba though is a comedy, whereas Re:Zero is a much darker, more psychologically realistic series that's showing us in bloody close-up what you or I might be like if actually in such a situation.
Admittedly Subaru generally wins in the end, but only after an agonising process of trial and error. This will involve trauma, bad decisions and the ugly deaths of himself and his loved ones. Oh, and the usual fantasy romantic subplot is also being poked with sharp sticks. It's there. It's a key plot driver. Subaru's damagingly genre-savvy and had all kinds of expectations right from the start. However the show's also setting up big questions about Subaru, Emilia and whether certain people are making the right decisions and/or letting themselves be blinded by stuff in their heads.
That's a brave way to write your protagonist. I'd say it's going further than industry-reshaping deconstructive shows like Neon Genesis Evangelion or Puella Magi Madoka Magica. However the show as a whole is less distinctive, with the worldbuilding being dark in a much more conventional way. It's quite an interesting world to explore and I liked it, but there's nothing too surprising there either. This is a fantasy world with lots of sympathetic people and a fairly sophisticated system of government, but also dragons, demons and assassins. Life can be frighteningly cheap, especially when you're getting involved with heirs to the throne.
Similarly the plot basically boils down to what you'd expect, underneath. It's a solid, well-told story of the kinds of people, spirits, ground dragons and giggling psycho death cultists. Our heroes do what heroes should do. It's just that Subaru often has to go through a few sanity-grinding failure loops en route. Oh, and it's based on a still-ongoing light novel series, so the finale doesn't tie things up as completely as I'd been hoping. If they do adapt the whole thing, then: (a) this will only have been its first sixth, and (b) I'll be very surprised. (It's good and I'd watch it, but the anime industry has a habit of abandoning great shows after only a season or two.)
Its conclusion is still good, though. The necessary things have been done, we have a happy ending and Subaru has made a clear choice that's consistent with his characterisation and inviting fierce audience discussion.
I'm glad I've watched this. I'm not sure if I'll ever feel the need to rewatch it, but it's clearly both ambitious and good. It's meatier than most anime, while also usually being fun and entertaining. (It's not a dreary misery-fest. Its darkest hours are pretty damn dark, but that's just part of the show's range.) Besides, the resurrection loops make for an unusual story structure. Admittedly there are other examples, e.g. the Japanese light novel series 'All You Need Is Kill' that became a 2014 Tom Cruise film. However I haven't seen or read either of those, so to me this felt fresh.
I also like the characters. Subaru's thoroughly nice and trying really hard, even if he's capable of being an obsessive, self-destructive quasi-stalker who makes decisions while he's hysterical or in an otherwise unhealthy psychological state. He makes some good friends. He earns his victories (and how). I presume it's one of the landmark anime of 2016.