From a fanboy standpoint, I can see why they didn't like it. It is for all intents and purposes, a musical. It just happens to be a musical about a character that was much better illustrated in a movie five years ago. I guess that's my biggest takeaway from it: it really makes me appreciate the first movie a lot more by comparison. As for canon, it's relation to Batman is just about zero. Gotham and Arkham are mentioned, and the prosecutor is Harvey Dent, but while the first movie gave you a definite feel for the Joker/Batman world, I can see why fanboys would stay away from this one in droves. You could have easily changed the names of the characters and its setting and not have missed a beat. And I don't mean that in a good way.
Kudos to the director and screenwriter for taking chances, I'm just not that impressed with their results. YMMV.
I'll add that I was the only one in the theater at the showing I chose. It was an early afternoon showing on discount day, too. Those tend to attract more folks given the price of tickets. I can't believe that bodes well for the movie's bottom line.