On Twitter, I noted that American Gangster was good, but seemed like just a very long episode of Kojak. It's obviously more than that. But on a character and plot level, I still think that's all it is.
Despite being three hours long and covering a number of years (a decade?), I don't recall any significant character development in either Frank Lucas (Denzel Washington) or Richie Roberts (Russell Crowe). About the closest thing I see to that is that Richie suddenly decides that he really is a bit of an SOB and lets his ex-wife take their son where she wants. And Frank, once he determines he's got no other option, suddenly decides to play along with the good cops, in order to take down the bad cops and the mob. But that actually seems like more of an appendix to the rest of the film. I suppose that's what one gets in a long police procedural based on actual events.
Otherwise, this is an amazing period piece, deftly acted, and smoothly directed. I totally bought the grimy New York underbelly of the 70's, the junkies, the crooked cops, the actual grime. Recommended.