The key reason why it sucked so much was that there was no depth in...anything.Everything was spelled out. It was like M. Night said "Zuko's the bad guy; Iroh's a nice guy; Aang's the last Airbender, and Katara's worried all the time. Enjoy the rest of the movie." ...
As it looks like things are over for team Avatar, Uncle Iroh jumps in and defends Katara and Aang. He tells them to run, and he holds Azula and her team off in the process. After Aang has successfully escaped, Uncle Iroh gives himself up and is taken away as a Fire Nation traitor....