For me, it was right after the Pain Arc, where the story started to loose its main focus. It changed the main antagonist of the story from someone with an interesting background, to just another Uchiha man child.
The war arc that followed soon after was just a nail in the coffin. The main character became overshadowed by a secondary character who I am now convinced was a self insert.
It is also at this moment where most of us should have seen it all coming. Kishimoto himself admitted he made Madara too powerful and thus had no idea on how to deal with him. At this moment we should all have been thinking that the writer was just pulling things out of his behind, and had lost all coherence.