A lot of story flaws and silly ideas.(Picard, a senior citizen, going off to fight alone on the Scimitar, for example) Shinzon never really works as a supposed "dark mirror" for Picard as they are nothing alike. The movie seems to ignore developments in DS9 about Worf and the Romulan Empire.
However, the movie's not really that bad. It's an improvement over Insurrection, and mostly suffers in fandom from coming out at a time when the franchise was in serious decline.