Firstly, he is very one-dimensional and lives for only one purpose - to kill Itachi (he even said so himself). Now, people can make the argument that other characters like Rock Lee and Neji are one-dimensional as well because they too only strived for one purpose: to prove that someone can be a splendid ninja with taijutsu alone and to rebel against the idea of subservience. But this is where they're wrong: even though Lee and Neji's goals can be viewed as being straight-forward, the very essence of their objectives coincided with one of the overall themes of the story itself (perseverance). Sasuke's one goal, vengeance, does not coincide with any of the values that the manga tried to push. Basically, not only was Sasuke one-dimensional, but his only goal didn't contribute anything to push the story forward.
Secondly, Sasuke is very unoriginal. He is supposed to represent an "arch-Naruto" had Naruto internalized his pain of loneliness and festered it into hatred. The problem with this lies in the fact that it's already been done with Haku first, then Gaara (the best "arch-Naruto"), and lastly Obito. The other reason why he's unoriginal stems from his newly formed desire to destroy Konoha after Itachi's death. Again, we've seen other characters strive for the same thing: Orochimaru and Pain (who actually succeeded, thus making Sasuke's mission void of any validity). Once Itachi died, Sasuke's journey was over. He had no purpose left within the story. If I was Kishi, I would've had Sasuke die together with Itachi. But by forcibly extending his worth past it's expiration date, not only does the story suffer (repetition of generic goals), but other characters suffer as well (the return of Sakura's infatuation).