For me, the biggest problem with NEM is that Shinzon's motivations are cloudy. We're told he self-identifies as a Reman and is a Reman nationalist... but out of nowhere, we discover that he hates Picard and wants to destroy Earth.
I mean, I guess I can understand the idea that he'd irrationally come to hate Picard. But the idea that he'd turn his genocidal rage against Earth instead of Romulus as the end of his life approaches just doesn't make sense. Romulus has been oppressing and terrorizing the Remans for centuries; as a Reman nationalist, Shinzon's goal should be to destroy all life on Romulus before he dies.
And the purple rubber outfit. Seriously, whose idea was that?
Also, it would've been a very good Trek moment to have Picard helping defend Romulus from Shinzon just because it's the right thing to do. The fact that'd he put his crew's life on the line to fight beside their enemy could've been a big step toward peace with the Romulans. Also, why not have Picard get through to him at the end? That might've been a nice way to end the TNG legacy.