All kinds of stuff. Disclosure about what the U.S. diplomatic staff knew about corruption and torture in various countries -- and what they really thought about the ruling elites of these places, despite public statements to the contrary; attempts by companies such as Chevron, Lockheed-Martin, and Coca-Cola sought influence in certain countries; how the U.S. arranged to spy on UN officials, etc.
Here's one of my personal favorites (summarized by Reuters):
"You know the movie 'The Godfather'? We've been living it for the last few months," a businessman involved in the dispute was quoted in the cable as telling an official from the U.S. diplomatic mission in Tripoli.
The cable, which was made available to Reuters by a third party, centers on a bottling plant in Tripoli that was shut down for three months. It had been seized by troops loyal to Mutassim Gaddafi, a son of Muammar, who at the time was feuding with one of his brothers, Mohammed. (Another State Department cable suggests a third Gaddafi son, Saadi -- better known as the family's professional soccer player -- may also have been involved in the squabble, though no details of his role are given.)
Eventually, the American diplomatic mission in Tripoli, known then as the U.S. Liaison Office, sent a firm protest to the Libyan government. The document states that around the same time, Mohammed Gaddafi, possibly under pressure from his sister Aisha, a family peacemaker, apparently agreed that shares owned by the Libyan Olympic Committee, which he led, would be sold to a third party.
Shortly afterward, the cable says, Mutassim's men left the Coke plant, ending the family standoff, but not before employees of the plant received threats of bodily harm and a Gaddafi cousin was stuffed in the trunk of a car.
Really, it shouldn't be hard to satisfy your own curiosity on this topic. Unless you think we're better off not knowing about this stuff, that is.