One thing I find hard to believe with Nemesis is that it took Troi and Riker 4-5 years to get married after getting back together in Insurrection. I kinda wish the film had had them married and already onboard the Titan
, I'm sure the audience could wrap their heads around the passage of time and off-screen character development.
As for who dies at the end, I was alright with Data sacrificing himself for Picard. But he should have stayed dead, the whole B-4 thing was such a cop out and should have been removed.
There are lots of other little bits that should have been changed or explained, but the biggy has to be Data 2.0.