It's been pretty widely observed that Barack Obama's relatively brief career in elective office hasn't actually included any tough races against Democrats. Less noted, but also true, is that John McCain's rather long career in electoral politics hasn't included any tough races either. Like Obama, McCain's toughest fights have come in primaries -- specifically his successful 1982 congressional primary, his unsuccessful 2000 primary, and his successful 2008 primary.
In part as a consequence of these realities, and in part as a consequence of the particular dynamics of the 2008 primaries, very few people have heard McCain criticized from the left or heard Obama criticized from the right. That's an unusual situation, and it means that public perceptions of both men could prove quite fragile and prone to shifting once the battle is joined in earnest.