Tags concede defeat, fragmenting one dysfunctional community into a whole spectrum of subcommunities. How will we handle the tag for "good enough for '08 Hacker News"?
If you can sideline all the people who want to bullshit about how the ipad2 is the future of personal computing or the quality of the writing at TC, you've already won.
No, what Reddit does is fragmenting community. How much does the tag system in StackOverflow or Quora fragment them? Tags helped people to somehow filter their already overwhelmed input stream, focus on what they really interest in, and more importantly, ban the items they genuinely don't like to see.