Nobody intelligent believes in full transparency.
Go back and read that again, to let it sink in once you get past the "OMG this guy is just trolling" knee-jerk.
If there can't be secrecy in negotiations, the people we try to persuade to change or deal will instead pack up their toys and go home.
Quite frequently, the entire key to a diplomatic deal is exactly that it doesn't appear as it seems. Hell, that's made clear in these leaked documents. They make it perfectly clear, for example, that the situation with Libya would be much worse if things weren't done in secret. Since they explicitly made concessions in exchange for asking that we say something nice about them in public.... They couldn't have even asked for that if we had total transparency. So what would we have had to give them instead?
Maybe Assange is a tactical genius, and selectively revealed only the documents that he knew wouldn't cause war. Or maybe he's lucky. The stuff he released could easily have sparked World War 3, between the middle-east and korean/chinese revelations. We may never know which (or we might still end up at war in Korea).
So be careful before you lump everybody who stands up and calls for his head together with Sarah Palin. She's not always fundamentally wrong. She's just not sophisticated enough to understand the details, and she sensationalizes stuff 'cause she's a politician. She doesn't understand the consequences or side-effects of "using cyber tools to permanently disable WikiLeaks", but the overall sentiment is sound. Namely, we need to shut Assange up, and we need to shut his sources up, and we need to get control of the information that passes through our embassies. We just need to do it intelligently instead of invoking terms that we don't understand that amount to cries to nuke stuff from orbit.
I hate to say it, but Hillary seems to be handling this very well (and seems to agree with Palin on the basics).