Once a user knows the hash of a file in BitTorrent, downloading that torrent is easy, fast, and secure. But finding a hash is a completely different story; the hash must be downloaded from a website, which then becomes a single point of failure in the torrenting experience. The Pirate Bay, probably the most famous torrent index, is one of the most popular sites in the world; at the end of the day, many torrent users are putting their security in the hands of a few Swedish guys. If this is something that so many people care about, why aren't there decentralized torrent indices?
World-writable data structures like lists, sets, or maps could solve this problem, as well as many others. It would be possible to create censorship-resistant username registries, content directories, and domain name systems.
Currently, though, there aren't many good tools for building these shared decentralized data structures. My proposal is to build world-writable data structures using the Bitcoin blockchain. These data structures must be easy to use, spam-resistant, and censorship-resistant.
This sounds so simple... so why doesn't it already exist?