I wouldn't really say "Public Enemies" is a bad film - it just isn't a very memorable one. Here is why...
1. I now know why I am not a Michael Mann fan - While I appreciate his aesthetic, I have to admit that I don't really remember any of his films...Ali - I remember Will Smith not the film, Collateral - I know I saw it, but I could not for the life of me remember what it is about, Miami Vice - I remember feeling bad for Colin Farrell. It's a weird realization because he tends to have highly anticipated projects and he works with amazing people, it just seems like the final product always disappoints.
2. The trailer is so much better than the film - The trailer was highly stylized, cool, fast-paced and filled with well-known faces - Absolutely nothing like the film, which lost it's cool-factor after the first 1/2 hour, was drawn out, long and slightly boring and it gave zero screen time to anyone but Depp (except for one amazing scene featuring Marion Cotillard - proving that Oscar was well-deserved). The cast was incredible, yet Channing Tatum was in it for less than a minute (no joke, if you blink you will miss him), Giovanni Ribisi had like 4 lines, Christian Bale and Billy Crudup had 4 lines that they just kept repeating through the entire film and any other familiar faces in the film came and went so fast i stopped keeping track. Why cast such amazing actors if your not going to utilize them?
3. Everyone looked the same - It was really hard for me to distinguish who was who, not only because everyone had very little screen time, but also because they were all wearing the same thing. Guys were getting shot and I had no idea if they were bad or good or even main characters. I gave up trying to understand who was who, when towards the end of the film I was under the impression that John Dillinger had just been shot 5 or 6 times, only to have him reappear in a car - not shot (this was not just my impression - the person I was with thought the same thing). Obviously someone else important was killed, but I have no idea who. It's unfortunate because the film is called "Public Enemies" which led me to believe that the film is about several bank robbers during that era - not just Dillinger.
4. Johnny Depp is always memorable - I still think Johnny Depp was perfect casting, and if the script wasn't so limited - there probably would be Oscar buzz. It's unfortunate that we really don't get a sense of who John Dillinger was. The only thing we know is that he is a murderer, a bank robber and he can manipulate his women. But I thought I would get a little bit more information after watching it for 2 hours and 20 minutes.