I think the difference is that, if I recall correctly, StumbleUpon was more or less a recommendation engine, where anyone could submit interesting websites to be added to the list, whereas what I'm proposing is more akin to a search engine curated through individual websites' endorsements. The primary difference being that if you get bad endorsements, you can blacklist the source, so sites have a vested interest in providing good recommendations (and in turn, in theory, receiving recommendations back). It's basically how the web worked in the 1990s... you found sites via groups of links on other sites, not through centralized search engines, and those links were curated by other sites' owners.