The StumbleUpon comparison is apt but I think what made StumbleUpon work was the social layer: you could see what your friends upvoted, and that created an implicit filter against the pure randomness. Pure random discovery is fun for a session but gets old. Would love to see something like a lightweight trust graph here where a site vouching for other sites carries weight, similar to how Webring worked but with signal about quality rather than just affiliation.