A key difference between a Bush and Kerry administration would be their respective viewpoints on the nature of the Second Amendment.
For decades, the Justice Department held that the right to bear arms was a collective -- as in, "well regulated militia" -- right rather than an individual right. The first indication of a change in that position came in 2001. Attorney General John Ashcroft wrote a letter to the National Rifle Association's chief lobbyist stating that "the text and the original intent of the Second Amendment clearly protect the right of individuals to keep and bear firearms."
Under a Kerry administration, we'd get a new Attorney General and Solicitor General, both of whom would share the traditional collective-right view of the Second Amendment. That's one very immediate change we could expect.
It seems like those who are so strongly against losing the "rights" to their guns are the same who feel fine giving up free speech and privacy rights in light of the Patriot act. it just seems like these conservative gun owners are picking and choosing which rights they are comfortable giving up.