I like how people ask "are you voting for Romney or Obama?"
There are three branches to the federal government: legislative, executive, and judicial. You get three votes to influence the legislative, two for senators and one for a congressperson. Your other vote, for president, represents your opinion on the entire two branches of government.
When you vote for president you're voting for who you think should be on the Supreme Court, which judges should set on other federal courts, who should be the Attorney General and which cases should he file, who should be the ambassador to the PRC, who should be the treasury secretary and the chairman of the federal reserve, which advisors should be on the Council of Economic Advisors, . . . oh and who should gave the state of the union address and sign legislation from congress.
I prefer to think about the election in partisan terms because most of what you need to know about all these nominees is which party they will come from. Imagine if Charlie Crist, the moderate former governor of FL, run for president as a Republican. Who would he nominate to the court? Who would run his state department? Who would be on his Council of Economic Advisors? Now imagine if he changed parties (he probably will) and ran as a Democrat. Would they change? I think so. So the man who runs probably isn't as important as his party and that is why I'm a party line voter for president.