There's a lot of elements in Nemesis that could have worked. The conflict between Picard and a supposed younger clone of him was interesting if they actually went full out and made it about those 2. Instead Shinzon ends up being a typical villain that wants to destroy take over/destroy everything with no real good reasoning behind it. They really could have made it about Picard vs his younger self and had that as the only conflict of the film and made it much more interesting. Instead as a villain he Shinzon doesn't seem all that interesting after he starts mind raping Deana. They really could have done much more with the Picard vs Younger Picard route and made the motivations for Shinzon much less cloudy and had a solid villain. Instead you could have put any Romulan as that character and changed none of the movie.
They really could have also done something interesting with Data teaching his younger self about the roots of being what he is instead of having that character as a spy for Shinzon. So that plot point went no where in reality. This could have easily gone with the Picard vs Picards younger self and made it an interesting film overall.
I think the death of Data was handled mostly right. Doing it on his own decision without an emotion chip or any software deciding it for him was brilliant. In fact the emotion chip not being involved in the film at all is a good thing in this film and it kinda hurt the first 3 TNG films IMO. But it would have been better if they could have had a closing death scene like they had in the TWOK. Even though it would be copying too much from that movie. Having a closing scene with arguably the most popular character from the TNG era is needed and the biggest part missing from the movie. The deleted scenes of going through his quarters is decent but it still doesn't work overall. You need that final goodbye to the character for the audience.
Nemesis had good plot elements that really could have made a great film. But they didn't quite follow through on those elements. That's really what it comes down to. Think about the ideas that I just came up with and how it could have really made a great movie. TWOK as an example used pretty much everything to make a great overall plot with even subtle parts that worked with the overall plot.