Shinzon's motives were very confusing. He has such hostility and hatred for Picard, a man who has done nothing for him. He eradicates the Romulan Senate, but that's where his revenge against the people who created him and then dumped into a pit to rot ended. He should have been on a bigger rampage against the Romulan Empire, rather than Earth/UFP.
Then they should have dropped the whole "if you'd had my life you'd be just the same as me" and "I'm a mirror" crap. The nature-vs-nurture debate does not need a film made about it, especially not a Trek film.
Nemesis should have been the TNG crew's ST6, with them and the Romulans finally starting to make proper overtures towards peace.