The buggy sequence and the Reman ship invasion were pretty shit though.
You forgot Shinzon's death scene, too mundane to be properly effective.
The Reman ship invasion and Riker's fight with the Viceroy are IMO the reasons the battle feels too long. Cut them and the whole thing would be tighter and more exciting.
I've defended the buggy chase here before. The argument against it seems to be "It's TNG - it's not supposed to be exciting."

Compared to NuTrek it looks like a knitting contest.
Highlights:
Picard's deep conversations with Shinzon and Data are probably the best part of NEM.
The wedding was really nice too, a great way to reintroduce the characters.
I don't know how controversial this is, but I liked the glistening dark purple-and-green of the Romulan clothes and sets.
I guess you could just say his motivation was that he was a psycho and/or wanted to show everyone what a bad ass he was by wiping out earth. But that's really weak for a character as complex as Shinzon was supposed to be.
I think Shinzon was a complex and very conflicted character. Apart from clone-envy, his main issue was wanting to get back at his oppressors by being "more Romulan than the Romulans". (A little like a certain Austrian corporal.) Both of these are more unusual motivations than the standard ambition or revenge.