Joaquin Phoenix gave an Oscar-worthy performance in a movie that at first I just thought was okay, but the more I think about it the more I like it. Some critics complain it is a movie about nothing, but that isn't really true. I suspect some don't like it because they perceive a certain kind of political message (anti-rich, or that society creates villains) but the message is not that clear or simple. A movie that encourages discussion is a good thing. It doesn't glorify the Joker but does provide his point of view and prompts the question: if my life was a sick joke, would I have a breaking point, too? As an aside, this movie is clearly an homage to two other Robert DeNiro movies: TAXI DRIVER (1976) and THE KING OF COMEDY (1983), both directed by Martin Scorcese, and one could write an article on the parallels.
It looks like a fusion of THE DARK KNIGHT RETURNS Letterman bits and the KILLING JOKE origin of an initially sweet and innocent 80's "modern age" retelling of the Red Hood that was remade incredibly stupid by Alan Moore (like most of that huckster's remakes). The original Red Hood story was more like the Jack Nicholson origin.
Yes, there are plenty of Easter Eggs that are nods to previous sources -- Bruce Wayne even swings down a little pole at one point! JOKER is like a fusion of influences, all recognizable to old fans, and I think it works quite well. Of all the comic book movies I've seen so far, this one felt the most grounded as if to answer the questions: "what would this character be like in the real world and how exactly could that come about?"
Not sure about Jim Carrey since I was never much of a fan... I didn't like his Riddler. It felt like Jim Carrey and Tommy Lee Jones were both doing Jack Nicholson impressions in BATMAN FOREVER. I grew up with the Frank Gorshin version and I have a lot of nostalgia for that 60's BATMAN show (I was thrilled when Gorshin later did voice-over work on DIABLO II, a game I bought the day it was released). It was interesting how in the comic books a few years ago the Riddler was briefly reformed... a great idea, to make him a consultant detective -- second only to Batman in terms of being able to figure out puzzles and clues!