It's less polarising than the other DKR sequels, but there are still two very different viewpoints on this one. My googling suggests that serious critics like it, but that ordinary comic readers often call it boring, bland and with a very thin story.
For one thing, it's assuming too much reader foreknowledge. "Dark Knight Returns" is in the title because it's set in Miller's DKR universe, but it's a prequel whose Dark Knight does no Returning. "The Last Crusade" is also in the title, being the last adventure before the Caped Crusader's retirement... but a casual reader might not realise that, since the book stops before the inciting tragedy (the Joker killing Jason Todd). In fact, not many non-fanboys would guess that Robin's going to die at the end and that Batman won't rescue him. (Oh, and the "Caped Crusader" is a title one hears rarely these days and never in this book.)
It might even confuse hardcore fans, for whom the death of Jason Todd has since been unhappened and these days he's back in the Bat-family. (This is the DKR-verse, though. For Miller's Jason, dead is dead.)
THE NEANDERTHAL VIEWPOINT
Batman whines a lot about not being able to go on forever and gets hurt badly twice by Killer Croc. (That's not a trivial complaint. Bat-fans read Bat-stories to see Batman beating people up. That's what he does. This is like a James Bond story where 007 repeatedly gets his arse handed to him.) The baddie is Poison Ivy doing something fairly pointless that achieves nothing. The Joker mooches around in Arkham Asylum for most of the book, then escapes. (Why does it take him so long? He escapes from Arkham so regularly that you'd think he had keys.)
The dramatic events (Joker killing Jason, Batman retiring) happen after the final page. You'd be better off reading Starlin and Aparo's 1988 "A Death in the Family".
There's also a hilarious line if you've read "All Star Batman & Robin", which is also in DKR continuity. Batman says, "In many ways, Jason is like me... some ways that I'll take responsibility for. But I've never enjoyed this war. Never."
(Reviewer has to change his underwear, because he's just pissed himself laughing. Admittedly, though, Alfred also takes issue with it.)
BUT, ON THE OTHER HAND...
I really liked the Joker material. It's fresh to see him as a scary super-criminal in an Arkham that feels plausible. If I had a gun to my head and was forced to take the very very very very very very very undesirable job of running Arkham Asylum, I'd insist on this kind of security too. (I'd make it stricter, actually, but let's not quibble.) This is an understated, comparatively sane Joker, with even his hair colour being muted, but you don't often see him portrayed like this.
As for the Batman-Robin scenes, they're about character, not plot. The latter's also important, admittedly, because Batman's punishment at the hands of Killer Croc will mean he can't handle things on his own and ultimately write him out of the story. What matters, though, is Jason's tendency to go too far in combat and enjoy it. Furthermore, as Alfred points out, Batman's bad at handling this. He's clumsy when it comes to conveying his opinions and too quick to resort to laying down the law, to the point where it probably is his fault that Jason will charge off like that and get killed. You can see the faults of both sides and the emotional pressure that's building up. "He's not ready. And maybe he never will be."
Social commentary is almost absent, despite a brief reference to the ninety-niners. You could argue that there's a motif of Gotham one percenters (Batman himself and Poison Ivy's stooges) but the story's not really doing anything with it.
The big question, of course, is whether the story was right to end where it does. It's not what I'd have done, but it's a choice. Personally, I can see why people think this is a lacklustre sequel. Its choices are either unimaginative, predictable or robbing the story of its punchline. It does, though, have strong character writing and it's worth a look on that level. It's a million miles away from the crazy mental excess of the other DKR sequels and "All-Star Batman & Robin", but it fits well (tonally and artistically) with the original DKR and would be quite a good chapter zero for it. Next time I reread that, I'll immediately precede it with this and see how they work as a double-header.
Thinking about it, I can see two possible rereading orders for DKR-related comics.
#1: THE GOOD STUFF
Year One, The Last Crusade and DKR. In context like that, I think The Last Crusade would work a lot better. It completes the story of "rise, fall, return".
#2: THE ROBINS
Looking back at it all, I reckon Miller's Batman work is ultimately about Robin. He destroys his first two (Dick Grayson and Jason Todd), before ultimately redeeming himself with Carrie. She's still a presence even in the big, bonkers DKR sequels that have relatively little Batman in them and feel more like Justice League Returns. We see her growth from brat to Robin, then ultimately to Batgirl/woman.
You might thus consider skipping Year One, but it does set up the ongoing thread of Gordon's home life. After that, All-Star Batman & Robin is hilarious but also a must-read, being part one of the destruction of Dick Grayson. You might even throw in Frank Miller's Spawn/Batman here as an extension of Miller's Batman being a dick. (It's set in that continuity, after all.) Now you'd read The Last Crusade (destruction of Jason Todd) and the keystone of it all, DKR. Carrie appears. Next would be DKSA (Dick Grayson part 2) and The Master Race (the end). The Golden Child would conclude it, being all Carrie and no Bruce Wayne.
That's a more interesting story than I'd expected. You could do a lot worse... and The Last Crusade is a surprisingly mature entry in that list.