If the documents only harmed the $evil_guys_in_power, that would be one thing. But a mass release of diplomatic cables damages far more than just those at the top. It risks exposing confidential sources to those who would kill them. Ironic, considering Wikileaks' own reliance on confidential sources, and the need to protect sources from harm, don't you think?
In the case of China, the US, and North Korea, it could further destabilize a situation that is tenuous at best. North Korea has something like 10,000 mortars (which it apparently has no qualms about using) aimed at the heart of Seoul and its 12 million people. Can you imagine the carnage and suffering on both sides if things go from bad to worse? Assange and his supporters can say whatever they want, but the fact is that they have no way of knowing how much bad shit could happen because of the release. They are fucking with things beyond their knowledge and control, and are in over their heads.
You know what? If Assange had a cable that said "Hey, I just personally blew the head off a civilian I suspected of terrorism, signed, GW Bush" and published it, I'd be okay with that. Find some true evil and expose it. That's what Wikileaks is all about, right? Instead, he released information that isn't a big pile of $evidence_of_evil, it's just a collection of stuff that some other guy (with an anti-US-govt bias) stole. If what I read in the news is correct, it doesn't even have a big "US is teh evil" smoking gun. Of course, it does make it harder for our diplomats to do their jobs, because now the guys who talk to China, the Saudis, and others around the world have to convince them that their private chat won't end up on the front page.