The casting of Shinzon. The fact that it was supposed to be the final TNG movie yet they missed so many opportunities. A story about Romulus seeking peace with the Federation without Spock/Leonard Nimoy. Admiral Janeway. The very crappy, un-imaginary, low budget production design (that wallpaper in "Alaska", oh my god, making a desert look alien by using some color filter, aww, the awfully small sets (they even reduced the size of the engine room because of budget issues)), the B-4 subplot badly done (from that sudden positronic alarm on the bridge to the search on the planet (was the attack on that planet already part of Shinzon's scheme?) to the fact that nobody cares about giving that thing all of Data's memory, nobody thinks about Lore, to the crappy Katra-thing they pulled off), Data's death was horribly done (but so was the entire final showdown)... I could go on, and I haven't even mentioned the countless plotholes yet.
The entire movie needed an overhaul. I haven't seen something like that in my whole life. The Final Frontier was a much better movie than this. At least TFF had a heart & soul and was enjoyable. Nemesis has a lot of flaws beyond belief and has no soul.
And it could have been so good. It had so much potential. Being the final Star Trek movie, it was supposed to come full circle with the entire movie franchise, it was supposed to come full circle with TNG, that Romulan plot was great, Picard having to fight his own clone, who's part of an old Romulan scheme to infiltrate Starfleet... my god, that could have been EPIC.
But the writer and director totally fucked this thing up.
I better stop now, otherwise I'm going to ramble on the whole day.