First off, they kept talking about joker doing his act, but then when he came out he just went straight to the couch and didn't do an act.
2nd, he wasn't funny, he was actually pretty serious. Joker is always funny, even as he's killing people. He's usually not angry when he kills.
3rd, it turned cliche when he started blaming society and the system... Just lame stuff that we hear losers complain about all the time.
4th, it was anti-climactic. We knew he was going to shoot deniro, and he did. There was nothing really interesting or unexpected about it.
5th, I personally thought the dialogue was weak but that's just my opinion.