Well, there's a few things I personally think are a must for a config format:

1. It must be human readable and editable, in some way. - in many cases, you may want to go and change something in the config while the application proper isn't running. That rules out stuff like pickle or binary formats. Although I suppose sqlite and it's ilk still fulfill it, in a roundabout way.
2. It should be unambiguous, with one way to do something right. - this one's a doozie. JSON fulfills it since it's unambiguous about it's types, but many interpreted language configs will have options. And then YAML will have "no" turn into "false".
3. It should probably have comments. - handily failed by standard JSON implementations. Although to be fair a lot of parsers I've used understand comments. Or you can make a comment stripper real easily.
4. It should have obvious structure. - I've dealt with CSV configs before, I do not want to ever again.