I really don't like people who say "Oh, the Democrats and the Republicans are the same thing, man! They're both parts of the capitalist/government system, man!", because they aren't.
Romney would repeal Obamacare. Obama would keep it.
Romney would raise taxes on the middle class and the poor and cut them on the rich. Obama would do the opposite.
Romney would veto the DREAM Act. Obama would sign it.
Et cetera, et cetera.
For one, Romney used to be heavily in support of a lot of what Obama has actually done. Obamacare was Romney's idea. Romney used to support the rights of gays and women. Romney also, actually, was for affirmative action back when he was running against Ted Kennedy. Of course, to get elected as a Republican presidential candidate, he had to change all of these views so that they fit the conservative mold, but he doesn't mean a word of it.
Not to mention, they agree on foreign policy on almost everything, except Obama doesn't want to bomb the sh**t out of Iran YET, while Romney wants to do it right away. They both want to keep the Patriot Act alive and Gitmo open. Plus, even on the immigration issue, Romney never specified whether or not he wanted to revoke Obama's executive order.