TNR nails it:

Whatever the political calculations involved in picking a veep, the most important qualification for the vice presidency is the ability to assume the presidency in a crisis. Given that of the last 12 presidents, three have either died or resigned, this is hardly a hypothetical consideration–in fact, given that McCain is 72, it is a very real consideration. Harry Truman, Lyndon Johnson, and Gerald Ford all faced multiple foreign policy crises immediately upon assuming office, whether it was the onset of the Cold War, the North’s invasion of South Korea, the Vietnam War, or the withering of détente and the resulting increase in nuclear tension with the Soviet Union. The next president will have to finish the denuclearization of North Korea; prevent the nuclearization of Iran; organize a departure from Iraq that maintains some level of stability; defeat a resurgent Taliban in Afghanistan; establish, nurture, and make the most of a relationship with Pakistan’s new leaders; and confront a revanchist Russia while preserving and enhancing its cooperation on nonproliferation and climate change–to say nothing of working with India, China, or our allies.

Could Sarah Palin conceivably manage this task? Her tenure as a small-town mayor and Alaska governor has given her no foreign policy experience whatsoever. True, Obama has little foreign policy experience either, as McCain and others have pointed out again and again. But during his time in national office he has demonstrated a clear commitment to the most pressing issues in American foreign policy. …

[snip]

… By contrast, there is no indication that Palin has even shades of a foreign policy worldview; a Nexis search doesn’t turn up a single article that she has written on international affairs.

McCain undoubtedly thinks he has his national security bases covered; picking Palin shows that, unlike Obama, he doesn’t need an eminence grise like Biden to add heft to his ticket. But surely McCain recognizes that Palin may have to fill his shoes someday. By choosing her anyway, he has demonstrated hubris well beyond anything Obama has displayed on his most arrogant day: a belief that he can master unforeseen circumstances, physical and otherwise, that are well beyond his control. This is insulting and dangerous and suggests that McCain may want to think twice before accusing Obama of putting his personal ambition ahead of the national interest.

An Astonishingly Arrogant V.P. Selection – The Plank.