Ordered in decreasing relevance:
First, the characters feel as flat as in no other TNG story. This is partly due to the director, partly due to the editing (virtually all good scenes of the movie are the ones which have been cut out) and, this might be controversial, partly due to the actors. They have played these roles for years so they also bear some of the blame.
Second, vampires from the dark side of the moon taking over the mighty Romulan Star Empire is an insult to the Rommies who have been virtually totally absent from a movie which should have been about them. NEM and STXI missed the opportunity to tell a great story about one of the best and most underused Trek species.
Third, Shinzon's motivations are guessable (he views himself as Remna, hates his weak human side and project this self-hate upon Picard and humankind) but not actually in the movie. It would have been far more natural if he had tried to blow up Romulus, the homeworld of the people who mistreated him.
Fourth, Stewart ruined his character via his constant demand for more action. He might like fast cars but Picard doesn't and interpreting his actions as midlife crisis is dubious at best.