A couple of throwaway lines to explain details that are clear enough if you think about them, but often get nitpicked by the film's naysayers:
1) The mysteriously galaxy-threatening supernova
2) Spock and Nero's ships surviving their first encounter with the black hole
3) Nero apparently focusing on revenge rather than on saving his world
It would've taken only a line or two to cover each of those in greater detail than the film managed, I think.
Oh, I probably would've added a scene with Pike reminding Kirk, Sulu and Olsen that Vulcan's got higher gravity and thinner air than they're used to. That'd serve to make their fight on the drilling rig seem all the more impressive and badass.