To me this appearance on Larry King sums up why McCain doesn't come off so well. He is really bad at spewing nonsense the way many others on the right do. McCain, of course, as any sane person does, actually has reasonable views on things (we might disagree with him, but he's (probably) not crazy (I think...)). When his natural inclination to be reasonable collides with hysterical political slogans and mud slinging, McCain does a poor job of explaining himself.
When McCain did his best, it was just him, on a bus, talking with people. He barely even had a staff at the time. The trouble is, as many pointed out back then, you can't run a national campaign that way.