The a-story is about Tony's alcoholism and depression and father issues, and the b-story is about some guy who is angry. The a-story's big action sequence (the best of the film) is against his best friend. Then Tony cures all his psychological problems and flies off because he remembers there's still a b-story to wrap up. The b-story action sequence is very long, and poorly filmed, and darkly lit, and with a lot of bad CGI. It is boring and takes forever and, because it is the b-story, you know it's not very important. It deflates what was otherwise a decent movie.
Also notable is that Terrence Howard is replaced by Don Cheadle. Imagine being in a huge superhero franchise only to find out that not only have you been replaced by a Golden Girl, but not even one of the more well-known Golden Girls.