Although McCain seems more or less benevolent at heart, he has allowed his handlers to shape his policy to the extent that he often doesn't seem to know what it is and has reversed himself countless times, denying statements whose authenticity is trivially easy to verify via video recordings posted on the internet.
Most egregiously, though, he has pledged to continue Bush's horrible policies, and indeed has Bush's endorsement. That ought to be enough to condemn him as a successor, but apparently there are still enough people who either support Bush or see Bush's failures as the result of bad luck (rather than his own idiocy, ignorance, and malice) and are willing to give those policies another go.
He also reversed himself on torture; his firm anti-torture stance was one of his strong points early on.
Another early strong point was his work against corruption; he has effectively undermined himself in that area by associating heavily with industry lobbyists, who have written many of his policies.
The McCain/Palin ticket is the first in American history in which both candidates were found to have violated ethics standards before a national election.