Who does it leak information to? how? what information anyway? and why is any of this a problem?

For example, if you keep statistics on the largest file a user has, then it dramatically reduces the number of files that the user's "true" file can hide in.

I don't understand what you are saying. Are you claiming that Bitcasa is some kind of steganographic cloud?

and there a variety of subtle situations where the DMCA wouldn't help them

Like what? Are Bitcasa any more vulnerable than every cloud storage provider (other than Apple which, as you say, appears to be a special case)?