Here's a thought...
Put on your foil hat for a second. Let's say you were a government and you wanted your citizens not to use systems like Tor and FreeNet. Wouldn't flooding them with extremely disturbing porn be a great way to make sure these systems weren't used by anyone except CP wankers? Wouldn't it be a great way to get people to go along with outlawing them?
This is a technical problem. When you think about it like that, it becomes obvious that this is a vulnerability in the security/crypto sense. I'd state the problem this way: these networks are trivially vulnerable to a particularly devastating social engineering spam attack that renders the network virtually unusable by most people. Call it a social DOS attack.
Edit: I believe I can state the problem succinctly:
Design a darknet/freenet network that is anonymous, uncensorable, and yet is not trivially vulnerable to social engineering DOS attack.
Hard problem. Hard equals interesting.
Personally, I've spent quiet some time on the .onion network and never been ambushed by child porn. Don't you think your problem is solved on the .onion network the same way it is on the clearnet: with moderation on a website-by-website basis?
What I'm saying here is that there's an interesting unsolved problem and that this problem might be the thing that's blocking the adoption of these technologies.
It's also a critical mass problem. I don't think you could run that sort of attack against the Internet because it has over a billion users. Once the network reaches a certain mass, it becomes far less of a problem. The problem is that CP-wankers (and possibly attackers) instantly colonize darknets, rendering them quickly polluted before they have a chance to escape their nascent phase. Like I said in another post: you could apply game theory here.