1) Relatively high freedom
2) Variety in values/opinions (often due to freedom)
3) Secrets to be leaked
1 ensures the leaker's ability to leak, 2 ensures there is a leaker to leak, and 3 ensures there is something to be leaked.
Assange outlined his goals  a while ago in regards to exposing secrets, and I think Wikileaks is staying in line with those rather well. It essentially states that they'll leak whatever they receive, and try to enforce a kind of 'secrecy tax' on governments/parties/organizations that refuse to be transparent via forcing them into less effective means of communication and overall less use of technology.