It's probably the most iconic Batman story ever. It stands above everything. The character's history can be divided into before and after The Dark Knight Returns. Yes, it really did have that much impact. In 1986, it was a thunderbolt. Today, it no longer feels revolutionary, since its key innovations have since been stolen and done to death by the comics industry. This and Watchmen are the two books that ended the Bronze Age of comics and ushered in the Dark/Modern/Iron/Whatever age. Killing iconic characters and doing a final end for the Batman? Routine these days in Elseworlds and What Ifs. Batman vs. Superman? See the movie. The death of Jason Todd, the second Robin? Happened for real two years later in "A Death in the Family"... yes, AFTER this series was published. But he's since been resurrected and given his own series.
Grim and gritty? Today, do we expect anything else? In those days, though... admittedly, Dennis O'Neil, Steven Engelhart and Neal Adams had already been reviving some of Batman's darkness in the 1970s, but have you ever read any of those issues? I used to have some Neal Adams Batman collections. The art's great. The scripts are hilarious. Fans applauded this return to the character's roots, but it didn't rescue the sales figures. In 1986, Batman's sales had been falling for well over 20 years (albeit with an Adam West spike around 1966). 1985 had been an all-time low.
The Dark Knight Returns blew all that to atoms and paved the way for, well, everything since. But sod that. It's important history, but how does the book read today? Answer: it's still cool. I think I slightly prefer Year One, but this series is bigger and far more ambitious than that one.
1. It's full of explicit discussion (often from TV talking heads) of the ethics, acceptability and psychological problems of Batman and his enemies. It reflects real debates on, say, toughness on crime and harmful influences on children. This is all the more interesting for the question of how much of it is deliberate. The book's openly hostile to liberal bleeding hearts and people who say that the Batman's a psychopath as dangerous as the people he fights, but then again this Batman is explicitly a son-of-a-bitch who deliberately terrorises and hospitalises his enemies ("the seventh one hurts") and hates talk of criminals' rights. His worldview is military, he responds to the Coldbringer EMP by essentially taking over Gotham City and he'll get even more unhinged in later Frank Miller stories.
How much of this is Miller not realising how his personal biases come across? In those later stories... dunno. Here, does it matter? In this book, it still feels textured and intelligent. Right-wing idiots can be as infuriating as left-wing ones, although both kinds are dangerous. The Joker's psychiatrist defends him as the victim of Batman's paranoid vendetta, so he lets the Joker go on TV and use his own lipstick. Result: death of everyone in the studio. Yes, including the psychiatrist. Carrie's hippie parents call the Batman an irresponsible menace to society, but never realise that their thirteen-year-old daughter's running off to fight criminals at night. "Hey... didn't we have a kid?"
2. Of the Robins I've encountered, she's the best ever. By miles. No contest. Theoretically, I like Robin, but in practice (s)he often seems to get written as a dick. Carrie Kelley, on the other hand, is awesome and I revere her.
(In fairness, though, there are Robins I don't know. I've just visited Robin's wikipedia page and... goodness me. They must bud off each other like plants, or something.)
3. The story itself is fun and gives Batman some serious opposition. Society itself has become his enemy, with an anti-vigilante near-consensus in the media and yet also street gangs so uncontrollable that they hardly even seem to belong to our species. They call themselves the Mutants, although they're easily influenced. They even have crime quotas, i.e. a minimum daily/weekly number of beatings, killings, etc. Meanwhile, superheroes have been suppressed. (Again, that idea's since been done in other comics, but there's fun to be had there too. One day I'll reread this alongside Albion, for instance.)
4. It's an end to the Batman's story. Technically, that's no longer true, thanks to sequels of, um, a broader quality range: The Dark Knight Strikes Again, Dark Knight III: The Master Race and The Dark Knight Returns: The Golden Child. You can ignore the sequels as you read this, though. Stories need endings and this is a fitting one.
5. Sometimes, it's even funny. Alfred never stops being rude to Master Bruce, while even Batman himself has his moments. His friendly advice to the Mutant Leader, for instance.
6. Its political commentary can be playful, e.g. the American Hostages Guild. (That gag hasn't dated so well, admittedly, but terrorist hostage-taking was a big thing in those days.) It also has Two-Face launching a terrorist attack on Gotham's Twin Towers, in a series written in 1985.
7. I like all the characterisation. Harvey Dent, for instance, although it's easy to forget about his story in issue #1 by the time you've read through issues #2-4. Gordon and Sarah. (That's a lovely, understated link with Year One, although admittedly Miller hadn't yet written that story.) Batman's relationship with Selina can either be taken as semi-dating, if that's your preference, or as further confirmation that the nearest Batman will ever get to romance is when fighting the Joker. That's the Joker's view, anyway, in his murderous way. "Darling." (I'm reminded slightly of Michelle Gomez's Missy.)
And then, most of all, there's Superman. Clark and Bruce know each other. They're friends. They still talk to each other... but Superman's a boy scout who'll always do what he's told, even when the President's an idiot who'll end up bringing down Soviet nukes. I love, love this series's godlike portrayal of Superman. The world it's portraying is as big a challenge for Clark's beliefs as it is for Bruce's, after all. THE WINK. "Isn't tonight a school night?"
So, at the end of all that... is The Dark Knight Returns any good? Well, yes. Obviously. It is, though, somewhat bitty. It doesn't have the dramatic momentum of Year One or Daredevil: Born Again. A surprising amount of it is intellectual or internal, with the overarching story arc being arguably just the evolving relationship of Batman with himself and the world. In terms of actual enemies, he overcomes a different, unrelated one in every issue and the last one is basically him fighting society itself. (Note his choice of allies.)
Personally, though, I love it.