I'm not sure I think the fact that John McCain likes to curse at his colleagues or say things like "thanks for the question, you little jerk" when he doesn't like a question at a town hall meeting is, as such, a problem. But it seems to me to be of a piece with potential diplomatic fiascos like his 100 years in Iraq gambit.
Part of being President is that it's important for you to not suddenly and accidentally alter the declared policies of the United States of America just because you didn't eat your prunes in the morning and you're feeling a bit grumpy. The Bush administration has, for years, pursued what its critics (myself included) have characterized as an Iraq agenda focused on the desirability of securing a permanent military foothold in that country. To its credit, however, the Bush administration always recognized the sensitivity of the issue to the point where they would shy away from coming out and saying this. That there was the diplomacy, and part of being president is knowing how to do it. On the Senate floor, straight talk like "Only an asshole would put together a budget like this" (McCain to Senator Pete Domenici, 1999 or "Fuck you. I know more about this than anyone else in the room" (McCain to Senator John Cornyn during negotiations over immigration reform) is a faux pas that can be overcome with an apology after the fact. Those same instincts could, however, be a major problem if applied to the DMZ in Korea or the Taiwan Straits.
UPDATE: Meanwhile, what do we think Domenici did with the '99 budget?