It's Frank Miller's best superhero work. I don't even see much room for debate on that, although I do also love Batman: Year One. (Don't ask me to rank this against Sin City, though.)
It's a fall and redemption story, with Matt Murdock falling deeper and further than you'd imagine possible. His life's already screwed on page one, after which he goes paranoid and insane. Genuinely crazy. He thinks his friends are conspiring against him and he can have a telephone conversation with the speaking clock, thinking it's Foggy. He beats up a cop. #227's title page shows him sleeping in his apartment, having already lost his job and with the bank denying knowledge of his mortgage payments as the tax authorities freeze all his bank accounts. #228's title page shows him sleeping in the only hotel room he could afford with ten bucks to his name. #229's title page shows him sleeping in an alley alongside vagrants.
The Kingpin's learned his secret identity, you see. Karen Page sold it on page 1 to buy heroin. She was once Matt's girlfriend, but now she's making porn films in Mexico. The Kingpin's destruction of Matt is so complete that it's almost beautiful... as of course is the Kingpin's evil. He can buy anyone. Generals, cops, you name it. Nowhere is safe. That cleaner mopping the corridor is watching you on his behalf. A nurse might break your fingers and make you listen on the phone as she chokes a cop to death. They're everywhere. It's as if the Kingpin's bought the world and might have anyone killed for reasons that the victims couldn't even guess.
"The cab is driven off Pier 41 into the East River. Its safety belt and doors are corroded shut by a chemical process that is identical to rust. Murdock is drenched in whiskey. A bottle, open, is laid in his lap. The owner of the cab is beaten to death by Murdock's stolen billy club."
(That's the only story point that rings slightly false to me, incidentally. Matt had fallen so far by that point that there was no reason for the Kingpin not to put a bullet in him there and then, instead of getting elaborate. Daredevil had become a nobody. I certainly don't believe that dumping a corpse would have been any trouble for the Kingpin's organisation.)
There are actually three fall-and-redemption stories here. One is Matt's. Another is Karen Page's, which is different but comes from an equally horrible low point. (Bloody hell, Paulo. He's a nobody. He doesn't matter. But he's terrifying.) Then, finally, the third is Ben Urich's. It's smaller than that of the other two, but just as horrifying in its sordid simplicity. (Notice, incidentally, how Mazzucchelli departs from realism when drawing Urich at his most terrified. His face becomes a shard of broken glass.) "Don't even think of the name. It's so very important that we don't even think of the name."
I love the Catholicism, for a hero who's always made such a thing of being a devil. (It's set over Christmas, incidentally, but uses Easter themes.)
When these people return to themselves, it's emotional. The entire second half is basically one big holy shit moment. Nuke lives up to his name. I don't even mind the Avengers showing up and being godlike (literally in Thor's case). It's the ultimate Daredevil story. It's why I'll always love both Daredevil and Frank Miller.