I speculate that the type of structure is the major factor causing the quality of social interaction to diverge. Wikipedia is people collaborating to make something, and only incidentally communicating with each other, on talk pages and user pages and so forth. Slashdot (and all its ilk, including Reddit and HN) is primarily people communicating with each other on comment pages. (I disregard story voting here because they don't have the same process, but also because voting is a relatively trivial input for a user, whereas writing a thoughtful comment/profane flame takes significantly more effort.)
In a nutshell, Wikipedia's product is not communication between users, so the Greater Internet Fuckwad Theory (http://www.penny-arcade.com/comic/2004/03/19/) finds less purchase. Slashdot/Reddit/HN do produce such communication, and then micro-optimizations determine the course such communication will take, like the moderation details/ability to downvote/etc. That's my hypothesis, anyway.
In a nutshell, Wikipedia's product is not communication between users, so the Greater Internet Fuckwad Theory (http://www.penny-arcade.com/comic/2004/03/19/) finds less purchase. Slashdot/Reddit/HN do produce such communication, and then micro-optimizations determine the course such communication will take, like the moderation details/ability to downvote/etc. That's my hypothesis, anyway.