Links posted to reddit mostly come from elsewhere, too. There are vast swaths of original content, but that site remains a news/link aggregator at its core, just like Slashdot.
One of Slashdot's biggest problems IMO is a lack of timeliness. By the time something hits the front page here, it's often old news; not just by an hour but by several days. News has become increasingly social and people want to discuss everything, but attention spans are growing shorter, which does not bode well for a site consisting mostly of stale stories. I know that by the time I see a story here, I've often commented about it (and exhausted my mental tolerance for debate on the subject) somewhere else. Today was actually pretty decent with the stories about FBI fishing expeditions and police spy radars being showcased while the news was still topical.
I confess, I don't ever go to the firehose and vote on whether or not stories are any good. If the editors are relying on readers to push stories to some greenlight threshold then I guess I'll share in the blame.