The unique thing about k5 was that all the content was written by the users. Every other community on the web is just a link-n-blurb site with a discussion forum bolted on: Slashdot, Digg, Reddit, HN, MetaFilter, and on down the line. Only at k5 do you have long form, decently well-written pieces on any variety of subjects.
What made it work, for a while, was that the community voted collectively (and still does) on the submitted articles. There are two queues: the edit queue and the voting queue. You are expected to put your article in the edit queue and deal with community feedback for about 24 hours before moving the article to the voting queue. Your ability to satisfy the community often went a long way toward whether or not you were voted up or down. If your article made a certain threshold of votes (or votes plus sufficient conversation) it was posted, if not it was dumped.
Poor moderation and poor adminship killed the site. There were no (permanent) negative consequences for bad behavior, since a dupe account could just be made, and if it was banned it was no big deal. There were also no real benefits for good behavior. So after a certain critical mass of trolls and griefers assembled, they managed to drive everyone off the site from 2004-2006.
There used to be a list of sites run on Scoop software (the CMS that k5 is built on, a counterpart to 'Slashcode' running Slashdot). It used to be hosted at scoop.kuro5hin.org, but rusty let that subdomain fail about a couple months ago and hasn't fixed it.
DailyKos is the most prominent site using Scoop to manage frontpage content and diaries. They made a host of extensions, fixes, features, and changes, but apparently didn't share anything back with the main Scoop codebase.
It's not clear anymore where one can download the Scoop sourcecode, although I think some k5 users have upped a copy somewhere.
As far as I know, k5's still one-of-a-kind. The closest copy is a site called hulver.com ("HuSi" short for "Hulver's Site"). HuSi was created by a disaffected member of the k5 community, hulver, who got fed up with the trolls. It is much more focused on diary content rather than frontpage-quality long form content. Basically, the entire British portion of k5 fled to HuSi, and they have pretty active admins there who ban the occasional visitor from k5 trying to troll. Still, it remains very small, and doesn't produce the same caliber of content that k5 once did.
