I agree with a lot of the "best points" listed above--the Romulan Senate scene at the beginning was an insight into Romulan culture entirely left out of the TV series (understandably so) and achieved the precise effect it was after in the viewers. I really admired Data's memorial among the senior officers, and it seemed perfectly natrual for Riker to start the conversation about Data's life. The memory of Data he shared was a great choice, too--I am sure every single TNG TV series fan was yelling "Pop Goes the Weasel"! when Riker said he couldn't remember the song that Data was whistling when he met him in the holodeck. Shinzon wasn't a bad character either--he was played very admirably, but I do agree that an attempt to create a Khan/Kirk rivalry between Picard and Shinzon felt very far-fetched.
I think that maybe this film wouldn't have been judged as harshly, particularly by TNG fans, if it hadn't been the last film in the TNG film series. Because it was billed as their "final adventure," I think all of us had high hopes for it. I am sure that everyone who was/is a TNG TV series fan has their own favorite loose ends, and a film like this would be expected to tie at least a few of them up. I agree that the director's personal distance from anything TNG-related made it impossible to expect that kind of a result in this film, and that was definitely a missed opportunity. However, if this had not been the last TNG film, this may have been one of the best TNG movie adventures.