If Obama sells hope better than Hillary sells competence, it's not because people value hope over competence but because they value Obama over Hillary.... When unlikable people trying to sell themselves as competent make mistakes like switching positions on drivers licenses for illegal aliens in 120 seconds, or criticizing a candidate for his kindergarten essays -- it challenges their only claim to consideration, and makes them much easier to discount quickly.
Politics is about salesmanship, first and foremost, and if Obama sells his product more competently than Hillary sells hers, then so be it- that just makes him a better candidate.
There may be more to this concept, too. While a President's substantive policy and competence certainly matters, particularly on the foreign policy front and even more thanks to the expansion of executive power, I wonder if a President's ability to inspire is at least equally important.
For instance, if a leader is inspirational and makes Americans feel good about themselves and their future, might this not lead them to be more productive on at least a marginal level? Might this not lead to organic change on some level in which individuals do things for themselves and others that would otherwise be done by government?
Certainly, I think recent economic history would tend to back this up- there have, after all been 3 Presidents in the television era (which has brought the Presidency into our living rooms) who could arguably be described as inspirational: Kennedy, Reagan, and Clinton. Each of these three Presidents is associated with periods of sustained economic growth. Admittedly, this is not a large sample size, and correlation does not equate to causation. But it's something worth thinking about.
**UPDATE** Many thanks to Tyler Cowen for addressing my speculation here. His answer seems to be that personality/inspiration might play a role, but probably not. Which leaves my speculation as exactly that - speculation.