While the President of the United States is not a god, as you seem to want to be able to elect, it’s quite easy to see significant differences in the candidates.
In foreign policy, McCain and the Republicans have had the “bad nations are PURE EVIL and can not be negotiated with (unless we pretend they’re nice, according to, for example, the oil-for-morals program)” while Obama and the Democratic Party, while sometimes a bit fast to be caught up in the rhetoric of right-wingers, does not have as black-and-white a view of the world. This does not mean they can change the world overnight, but if you think cooperation and mutual understanding is a good value in foreign policy, then I’d go with Obama.
Domestically, I don’t see how you can really ask “what’s the difference.” The Democrats have traditionally been more fiscally responsible, often promote social programs to attempt to alleviate inequality, are not as much in the pocket of big business, they are generally more in favour of unions and government regulation of environmental and social problems that arise from predatory business practices. With Obama in the White House and a majority in Congress, there will be a strong push to reforming the health care system.
The Republican track record of messing with government for their own purposes is well known, see for instance http://www.quartertothree.com/game-talk/showthread.php?t=12663
In the next four to eight years, we will probably see some retirements from the Supreme Court. Now, the thing that usually gets attention is the argument over abortion rights, maybe not the complete overturning of Roe v. Wade, but there are the baby steps in that direction to consider, if you care. Though there are certainly other cases going up to the Supreme Court all the time, shouldn’t be that difficult to look up. If you live on the coasts, the abortion thing probably won’t affect you or the women you know, though.
On the other hand, it’s not necessarily that bad, 7 of the 9 were appointed by Republicans already, after all, and it’s not like the Republican party has been high-jacked by hardcore social conservatives.
The social issue is also pertinent in other ways. I mean, you can either have an anti-gay, anti-abortion candidate who cavalierly dismisses inequalities based on identity, or you can pick Barack Obama. Now, the president does not have supreme power over these issues, certainly, but let’s not pretend that that means it doesn’t matter.
Basically, I’d say that politics affect most aspects of our lives. That it doesn’t manifest itself in presidential storm troopers showing up outside your door forcing you to gay marry or a glorious leader making an appearance on television declaring that a crisis has been solved by him personally, doesn’t mean that the position of president is incredibly powerful, and that you have an enlightened self-interest in having someone who would promote policies you agree (or disagree the least) with in that position.