My memory is hazy, and I never rewatch anything past... season 5, I think, so I may be wrong, but it always felt like the last few seasons they were just throwing stuff at the wall and seeing what stuck. Like, wasn't Maura Tierny in one episode as Robert California's wife, who had a crush on Andy, and then just disappeared? And, all the stuff with the new characters felt awful and lame rehashes of earlier seasons. I find Ed Helms to be unfunny in, well, everything I've seen him in, so making him the center really blew up the show for me.
But, the final straw that broke the back was an intro where it made it seem like Pam was having an affair or cheated on Jim with some sound guy and Meredith commenting on it. I don't know what episode number that was, but I think that was in the last season, and I hated it so much that they even suggested it that I remember getting up from my couch, turning off the TV, and not watching any other episode after that. Also, the whole storyline with the one girl trying to have an affair with Jim.
It is amazing how bad these final seasons were. Are there some good moments? Sure, but as a whole, it's a train wreck.