So at this point I've spent considerable time on at least 3 different Minecraft worlds. My crew from work will occasionally seed up a new world whenever we see something collectively interesting in a new Minecraft release; then we'll play it for a while until we finish ruthlessly extracting every last drop of novelty, wringing the residue into our parched gaping throats; then we'll abandon it until the next interesting release requires a new world.
I am pretty confident we are not alone in this behavior. So this means it that there are probably tens or hundreds of thousands of worlds out there that existed, were loved, and are now abandoned. They represent untold hours of human intellectual and creative labor, but they'll either never be visited again, or -- worse -- the world files are entirely offline. This is sort of vaguely sad.
So I kind of want a Minecraft Tourism service.
Send them your old, your used up, your forgotten worlds, and they run them on a server. A mod to the Minecraft client would have a Magic Button that you can click to visit a random world. If you don't want to run the servers, you could probably just upload ZIPs of the worlds to Amazon S3, and just randomly select one from an index for the client to download.
I kind of love the idea of someone randomly logging in to my old abandoned world, like a blocky Indiana Jones or Samus Aran exploring old ruins, and stumble across weird and funny things like the Great Mushroomy Gift Portal of Doom we built, next to the patchwork nether star Beacon. Or my gratuitously large estate. Or the Infernal Shrine to Pumpkinhead on the Nether Road.
Visitors in this way still won't be able to recapture the actual experiences, like that totally hilarious time when you fell into lava but were prepared with a water bucket -- only to accidentally place it above you, sealing yourself into a lava pool with a layer of obsidian overhead.
But they'll be able to see the lonely echoes of it, and that seems pretty cool.