Obama and McCain are basically the same
How different would the policies of America's federal government be if McCain had been elected president?
A billion dollars has been spent this year to persuade people that there is a large difference between the parties/candidates. Voters with an existing preference for one of the parties also have the powerful in-group/out-group bias to contend with. There are not many advertising dollars spent convincing people that the parties don't differ very much (not zero dollars - some of the advertising by the smaller parties such as the Greens and Libertarians takes this angle), and there's no particular bias I can think of that might cause people to systematically err on the other side of this question.
This doesn't mean that the difference between the parties is small. It just means that whether the difference is small or large, it would appear large (assuming the advertising worked).
With this in mind, it's worth reconsidering the real differences between the parties. Is there any reason to think the difference might actually be small?
Patri Friedman brings up the spatial voting model:
In a two party-system, the parties will tend to converge towards the center, and thus have far more similarities than differences. This is because by moving towards the center they can pick up centrist voters without losing any of the folks on their extreme. They get farther from their extremists, but those extremists still vote for them because they remain closer than the other party. You can visualize this with a beach and two hot-dog vendors, and the rule that people on the beach will always buy from the nearest vendor. The equilibrium positioning occurs when both vendors are next to each other, with equal numbers of customers on each side. Also of interest is that customer repositionings that do not cross the center line do not affect optimum vendor positioning. 50% of vendor A's customers can move to the extreme end of the beach, and vendor A is still correct to be in the middle. This suggests that influencing the opinions of huge numbers of voters may still have little or no effect. In the hot-dog model, such situations provide great incentive for the entry of a third vendor. Unfortunately the majority rule of democracy makes this inapplicable, as a third party which splits the votes of an existing party simply guarantees that neither can win.
Bryan Caplan gets more specific:
Reps don't want to get rid of the welfare state. Almost all Reps support spending a big chunk of GDP on America's poor and old. And Dems don't want anything like socialism. Almost all Dems want America to remain a country where markets are the default and people can get rich if they play their cards right.
So what is the "key difference" between the parties? Rhetoric. When Republicans advocate a small contraction of the welfare state, Democrats claim that Republicans totally oppose the welfare state. And many Republicans oblige them by standing up for "liberty" and "responsibility." Similarly, when Democrats advocate a small expansion in the welfare state, Republican claim that Democrats oppose free markets. And many Democrats oblige them by saying things like "markets only benefit the rich."
This rhetorical illusion is so powerful that when a Democrat like Clinton adopts many pro-market reforms, Republicans still hate him as a 60s radical. And when Bush II sharply expands the welfare state, Democrats still hate him as a billionaire's lackey.