There happen to be many good reasons to vote for Obama over McCain, and I would probably make use of one if I were not currently disenfranchised. But there are even more bad reasons to vote for Obama, such as this idea that Obama is popular overseas while McCain is a Republican.
Take the example of France. As has been pointed out, though France overwhelmingly supports Obama, they made a different choice in their own curiously familiar election. A forceful, pro-American conservative from the unpopular ruling party battles a hopeful liberal in an election focused on transition and the economy. Sarkozy's success was based on his talking point that both parties want change, but conservative change is better and he can deliver it. Several people have pointed out that McCain needs to do something similar, and his convention speech was a step in this direction. But to the extent we care about the French, we should care about how they behaved when it come to a decision for which they bore consequences. On that choice, the French choose McCain.
But maybe overseas popularity is important because it suggests that people will like America again and bloodshed will stop. There are several issues with this. One, people just don't like America in general, as it's a unilateral power. People hated America when Bill Clinton was in office, and Obama signs on to the same platform of humanitarian intervention. Two, it doesn't seem like popularity among the masses matters as much as connections with leaders and geopolitics. That article discusses a bit how American policy towards Europe shows significant continuity between Clinton and Bush against popular wishes by local residents. Third, terrorists, like countries, behave on the basis of fundamentals, not Presidential identity. The candidate with better policies will face less animus. Last, Bush has been surprisingly effective in foreign policy his last few years. From improving relations with Japan, to nuclear deals with North Korea, to shoring up relations with China, to signing a Nuclear Deal with India--this is not what reasonable people expected to happen in Bush's lame-duck years. An Obama administration may very well turn out to be more effective on foreign policy than a McCain administration. But there are other issues at play.