Personally, I think this is enough to stop the spread (if not the storage) of horror. In other words, someone might safely store their cache on my computer without my knowledge (heaven help me) but I refuse to store anything that is searchable as a horror.
If you're fine with that, that's good enough; you can run this system, and political activists and perverts alike will be able to stick their blocks there, accessible to anyone to whom they can pass the relevant short keys. But many people will be uncomfortable with even this much.
> How do we design a system that is anonymous and un-censorable where users can opt out of being relays for certain types of data?
As long as we are reasonable by taking "un-censorable" to mean very difficult to censor and "users can opt out of certain types of data" to mean highly limit traffic of data type <x>, it seems like a hard problem until proven impossible.
How then can it be used to share anything? I can see how it could be used as a secure, distributed backup (which itself is rather handy) but I'm not sure how it can be used to distribute data.