Todd Gitlin puts it well:
TPMCafe | Talking Points Memo | Not Maverick but Erratic: McCain's choice of Sarah Palin is... his standard modus operandi. He's impulsive, erratic. Put him in a jam, he leaps from petulance to exuberant nose-thumbing. He may be old, but he's unseasoned--he's childish. He jumps outside the box and takes pleasure in his insouciance. Faced with a foreign policy problem, he thinks: Bomb.... Faced with Russia-Georgia-Ossetia, he thinks: Let's get the Cold War on. Bomb and drill, drill and bomb--this is not a steady hand at the wheel; this is a go-for-broke gambler playing the game as he loves to play it....
Obama was absolutely right to question his "temperament and judgment" in Denver. I noticed a special roar from the crowd at "temperament." Democratic partisans know what he's talking about. The problem is that much of the rest of the country doesn't.... Hilzoy put it well:
I was also struck by McCain's willingness to gamble not just with our country, but with his own campaign. He has chosen as his running mate someone he has barely met; who has no experience dealing with the kind of scrutiny she is about to face; who has, by all accounts, not been fully vetted; and who is in the midst of a scandal. That is a shockingly reckless thing to do. Obviously, I think it's worse to gamble with the country, but taking this kind of crazy flyer on someone you don't know nearly enough about is recklessness of a different kind, and worth noting in its own right.