McCain takes a strong interest in foreign policy, to be sure, but his main public appeal has been to endless remind voters of his history as a POW. On economics, he's repeatedly admitted that he knows very little. And on social issues, he doesn't even know what his own positions are. (See this hilarious report from last year.)
McCain is like Obama in that he's appealed to voters largely on the basis of broad themes and his personal charisma and history. The difference is that Obama is a former law professor who's actually done his homework on the policy, and McCain is still winging it.
Of course, this is St. McCain we're talking about, so rest assured he'll never be called on it.