Sasuke: The Kage Summit Arc frustrated me. It was as though he was developing hatred out of thin air as soon as he arrived at that Summit. Willing to kill now, okay, willing to abandon Taka now, okay, getting emotional in battle now, okay, etc. I just found it completely unrealistic. His development was all over the place in that arc.
And there's Itachi being a good guy. I would've preferred he died a cold and arrogant Akatsuki member. I honestly liked that Itachi. He was quite a menacing villain, and then as it turns out, he never even was a villain. It was nothing more than a plot device used to push Sasuke into darkness and turn him from a respectable person into a homicidal maniac. I still enjoy both Sasuke and Itachi's characters, though Sasuke's descent and Itachi's righteousness weren't all that believable for me.