I think the great appeal of John McCain and Barack Obama among the grassroots and on Main Street has much to do with their muscular patriotism. Both men exhibit an honest love of country, a wholesome affection that stands in stark contrast to the bellicose nationalism of the GOP and the Democrats' standard "America's okay" anemia.
They don't appear strictly beholden to party interests, nor are they easily lured into ideological crusades. McCain and Obama understand the presidency as a chance to unite and lead the whole United States, not just red states or blue states.
Their shared passion also explains why many thoughtful individuals are smitten by both at once, and why N.H. independents were neatly divided between their respective camps. An Obama/McCain general election really couldn't turn out badly: regardless of who loses, America would win.