Fridge Logic: Shinzon has a grudge against the Romulans for creating him, lumbering him with a genetic disease, and consigning him to a life of misery in the Reman mines. Therefore, he wants to utterly annihilate... er, Earth.
This was explained somewhat in the chunk of footage that was left on the editing room floor. Shinzon's lifespan was going to be pretty short even if he had succeeded in stealing Picard's blood, and he in fact killed most those responsible for his creation when he took out the Romulan Senate. His main goal was in fact to go down in history by destroying Earth, which would bring about the collapse of the Federation government and pave the way for a full-scale Romulan invasion. A bit of a hackneyed motive to be sure, but it makes much more sense than what we got.
His talk about the Remans being a race "bred for war" implies that he and they are basically a bunch of Blood Knights who want to start a war, this time on their own terms and not as slaves of the Romulan Empire. By extension they can be read as trying to show the Romulans up, taking out the Federation in one shot after centuries of similar Romulan plots failed. It's still pretty vague, though.
The Enterprise received six positronic signals, but only one of Data's components (his postrionic brain) would register such a signal. Each of B-4's parts also apparently has its own power source, if the "zombie hand from the sand" is any indicator.
Fridge Logic: Why isn't Worf the one to go on the suicide run on the Reman ship? He lived through the last big war, and his religion actually requires him to seek a glorious death. In the other movies Picard being the one to go in is due to circumstances or necessity, but here it makes no sense.