McCain's problem was not his arguments per se. It was his attitude towards Obama. Yes, Rove has clearly directed him to be dismissive and superior towards the man. It's supposed to make McCain look like he's above Obama; that Obama's a young and contemptable upstart.
But "that one"? Er, no. Not only did that bring up some rather unsettling thoughts in terms of the racial dynamic on the stage, but it made McCain look like an ass. Not that he isn't, but you aren't supposed to look like one in front of the American people!
And, yeah, I know about the "people saying 'kill him' at Republican rallies" stuff. What's sad is not that they're doing that to Obama, what's sad is that they'd be doing that to ANY Democrat. That's what conservatism does to you; all the bullshit about "Bush derangement syndrome" aside, it's inevitably conservatives who become the vicious and hateful ones. Not all conservatives, of course, but it's a notable tendency.