The problem is that Bush's foreign policy has been a disaster. He's alienated the US from the rest of the world, he's created a perfect recruiting tool for terrorists in Iraq, and he refuses to talk to people he doesn't like, therefore letting misunderstandings build and build until something has to give.
McCain offers exactly the same type of "diplomacy" as Bush. Essentially, do as I say or we'll bomb you to death. To me, that sounds exactly like the mindset of the terrorists, and makes me very worried about the future.
Obama will surround himself with people who have more experience, Presidents don't operate in a vaccuum. He'll be willing to listen to opinions that disagree with his, and will be able to make rational, intelligent decisions based on ALL the evidence. That sounds more like a leader than what McCain offers.