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.
The book was a famous bestseller in its day and indeed in its year of publication(in the 1920s) created a wave of interest in what was, even then, a bygone era and was highly regarded by the likes of Jorge Luis Borges. Scorsese had wanted to adapt it since the 1970s.
Out of the 5 actors nominated for the Best Actor Oscar that year, Daniel Day-Lewis was the one who received the most prior awards for his performance, winning several awards (including the BAFTA and the SAG). Coming into Oscar night, Day-Lewis was the likely favorite, and yet he didn't win.(Though as he lost to Adrien Brody in The Pianist, the loss is justified.)
Awesome Ego: There are two occasions when Bill responds to a cheering crowd with a dismissive, even bored, royal wave. Rather than coming off as pretentious or pompous, his arrogance comes across as so totally justified that the moments appear on the Moment Of Awesome page.
Draco In APimping Top-Hat: Bill Cutting. Not that anyone else in the movie is a good person, but for some reason Bill gets lionized above others due to his Badass factor.
Harsher in Hindsight: A lot of the anti-immigrant viewpoints of Bill's gang and the Know-Nothing Party are eerily similar to anti-immigration rhetoric in America today.
Hilarious in Hindsight: Daniel Day-Lewis played Abraham Lincoln in Lincoln. Extremely hilarious when you think about Bill the Butcher shouting "Down with the Union!" during a performance of Uncle Tom's Cabin and throwing a knife through a picture of Lincoln earlier in the film. Made even funnier with the fact Day-Lewis was nominated for Best Actor Oscars for both roles and won with Lincoln instead of Gangs.
Also, Bill's rival in this movie was played by Liam Neeson, who not only was the original choice for the role of Lincoln, but gave a notorious Take That against Day-Lewis and his Method Actor style in an interview, claiming that he could rely on his own acting talent for a role and didn't need to "live like the part" the way Day-Lewis is famous for.
The plot of the movie could easily be described as Kill Bill.
Magnificent Bastard: Bill the Butcher. Particularly remarkable because Bill does not display the flawless complex planning or subtle manipulation that are normally associated with magnificent bastards. In fact, it is quite clear that he knows he's fighting a losing battle, and will go down fighting anyway for his principles, but is also a hypocrite with some of those same principles. Despite these significant failings, he is such an electrifyingly charismatic character that he manages to inspire awe rather than disgust. All kudos to Daniel Day-Lewis for managing to pull this off so gloriously.
Narm: Amsterdam's narration that the New York of his time was more like "a furnace where a city might someday be forged" comes off as an incredibly contrived and cheesy bit of exposition.
Also McGloin's somewhat ridiculous declaration of racism: "THERE'LL BE NO NIG-NOGGERY HERE! NONE!"
In the fight at the beginning of the film, Hell-cat Maggie somehow manages to leap high enough into the air that she can swoop down on an unsuspecting enemy and bite his ear off. One thinks of Batman: Arkham Asylum.