Ron Paul could take the youth vote, so Obama would take it back by campaigning on student loans and Obamacare, both of which are awesome for young people. Herman Cain might maybe take really religious blacks, but Obama would still win them for being on the top of his ticket. Ron Paul's ideas would be examined. Clinton would have a field day debunking Ron Paul's economic policies (like the gold standard thing) on the campaign trail, while Ron Paul would be given the national spotlight to propose them for the first time. It'd be an interesting race.
But Obama would win a landslide, for a couple reasons. One, Paul won't get a cent from the super-PACs that supported Romney and gave him an advantage. He might get one plus the grassroots donations, putting him even with Obama. Also, the Republican base would not support Paul. Paul could appeal to their religion, but social issues (marijuana) plus foreign policy (he's no hawk) puts him at odds with his party. Plus the old white southern racists wouldn't put Cain a heartbeat away from the presidency, not with Paul's age being what it is. Obama wins a landslide.
I'm going to ignore the thing about Cain being black because there's so few people in this country that are like that it wouldn't affect the election at all. Plus if you're really so racist you don't want a black VP I think you'd jump at the opportunity to vote out a black president.
I think it's a little sad that people think so lowly of young people that they believe they would vote for a candidate based on "we'll give you stuff". That wouldn't work. Maybe for some but not for all. Same with women and abortions. I doubt that student loans would make more than a dent in Ron Paul's following. Many forget to factor in something critical: with Ron Paul leading the ticket, no matter who's on the bottom, the voting demographics would be very different. A LOT of people who didn't vote would turn out and few of those that did turn out wouldn't. Factor in all the disaffected people who didn't vote and I'd bet 80-90+% of them would break in Ron Paul's favor. That's why I think he'd win, potentially in a landslide.