I get that people want to reserve some judgment but I also get why people are hating this. A lot of us expected an amazing comic book accurate Joker for a change. Just like Batman we haven't really had an accurate life action Joker, they've done some great versions, but I hoped that, given how amazingly Marvel has pulled off comic book accurate characters, they finally realized that us fans don't want the "director's version" of a character, we just want the character, in a realistic version that fits the universe, yes, but the character in essence, not this gangster looking/shiny suits wearer/flashy Joker.
With all the leaks we realize the director doesn't really care at least for a visual accuracy at all, and while Jared Letto is an amazing actor and will no doubt do the best he can with what he's given, you don't really have a Nolan-Ledger combo, where the director gives the actor some great lines and setup and the actor is free to passionately perform. I saw something similar with Man of Steel. Henry Cavill does amazingly well. He is the perfect Superman and he definitely nails the nice-but-still-intimidating personality that Superman is supposed to have, but the movie never really gives him one single spot to really shine.
Hopefully I'm wrong and hopefully this pulls off a Guardians of the Galaxy where the changes are so welcomed that even translates into the comics, but I just don't see how that could happen right now.