These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: The Killing Joke
Epileptic Trees: During a 2013 podcast with Kevin Smith, Grant Morrison argued that the end of The Killing Joke in fact has Batman killing Joker, carefully interpreting the panels as evidence. He admitted that it was done in a way that was left to viewers. A very dark interpretation of the story but not unexpected from Moore. Artist Brian Bolland disagrees though, the comic's inclusion in the mainline DC canon clearly indicates it didn't happen, and the script for the comic says Batman and Joker just collapse helplessly in laughter. Though considering Moore's own ideas expressed in Whatever Happened to The Man of Tomorrow?, the fact that it's canon or not would have little impact on his own vision for the story, which as of now is not known.
Jerkass Woobie: Believe it or not, the Joker in the final scene. Physically and psychologically defeated, he kneels on the ground, listening to Batman's offer of rehabilitation and, in one Beat panel, appearing to seriously consider it, before saying sadly that it's "too late for that. Far too late," suggesting that there's a grain of sanity and humanity in there somewhere, albeit not enough to make a difference. Thus, the work shows the Joker at his single most sympathetic and humane moment.
Misaimed Fandom: There are those fans who, even if they think he is still a sociopath, believe the Joker when he says that one bad day is all it took to drive him over the edge (and by extension, that one bad day could drive anybody over the edge). Not only is this arguably disproved by the end of the story, since he fails to break Gordon, note though possibly because Gordon's day was several measures less bad than the Joker's might have been - we don't know how he would have reacted had Barbara been killed outright it's suggested by Batman that his failure and behaviour mean that the Joker was not even right about himself, and by extension needed help long before his Trauma Conga Line. That's not even getting into his Multiple Choice Past claims.
Batman: So maybe ordinary people don't always crack. Maybe there isn't any need to crawl under a rock with all the other slimey things when trouble hits. Maybe it was just you, all the time!
Drawings of Awesome - four different pieces of art from this work have become iconic for The Joker - #1. The cover image of The Joker aiming a camera at the reader, #2. The shot of Joker behind the door Barbara opens just before he shoots her, #3. Batman and The Joker laughing together in the rain (the middle panel), and especially #4. The Reveal of the Joker after coming out of the chemical bath, with the background filled with his laughter.
Moral Event Horizon: What the Joker does to Barbara. Of all the crimes the Joker has committed - even the murder of Jason Todd - this is the one that haunts the whole DC Universe.
Tear Jerker: This is one of the very few Batman stories that can make one feel sorry for The Joker, even after the horrible and nightmarishly evil things he's done this time.
Especially the scene towards the end, when Batman chases him. The Joker's face as he asks "Why aren't you laughing?" just breaks one's heart.
They Wasted A Perfectly Good Line Art: Brian Bolland's opinion on the original version. He subdued the color palette considerably and added a few extra color details for the 20th-anniversary edition.
Which in itself divided the fandom, with some arguing that the garish, acid-trip colours of the 80's original contributed to the nightmarish feel of the book. Tellingly, DC continues to publish reprints of the book with its original colouring.