The leak is bad, for the same reason the Presidential Records Act is bad, as currently written and construed, because it doesn't change the thoughts, opinions, or actions of those who write cables, but DOES change what gets written down.
The Obama Team thrives before entering the White House on Instant Messaging - now, since all communications must be archived, they must rely on face-to-face conversations, which causes inefficiency, among other problems. In the same way, the leak of these cables will cause inefficiency in three ways.
First, diplomatic cables will irrevocably be written with a fear of possible public consumption, meaning that true nuance will be sacrificed for political considerations, and that those writing them will have to use language that is less explosive when explosive language is required. Second, the leaks will cause more reports to go back to the State Department in verbal phone calls, or at levels of classification (Top Secret) that aren't fitting of the situation. This, and most worrying, is that this will make it harder for someone in Uzbekistan to look up State Dept info on another country that's relevant to his work in Uzbekistan - we need information to be able to flow freely in the classified system, and because of this leak, it won't flow nearly as freely.
As for the content, I find them interesting, but there are no revelations that justify the problems caused by the release - most all of this stuff was easily guessed without official confirmation, and the private stances of foreign officials are PRIVATE for a reason - making them public won't change their public stances at all.
