Registration and Unintended Consequences
Usually the law of unintended consequences decrees that things will be much worse than expected, but every now and then there's a pleasant surprise. Story from Topix via Greg Linden. I won't rehash the story in too much detail, but the upshot is that Topix got rid of their registration requirement on their user forums and not only did participation increase dramatically (as expected), spam rate actually decreased (pleasant surprise).
The thing is, requiring registration has some unintended consequences of its own. In particular, lots of people won't do it. And, on the other hand, spammers, trolls, and immature jerks are happy to do it. So (proportionately at least), you end up with more crappy posts. These observations are from the "2ch principles"; 2ch is an anything-goes web forum in Japan, and people have modeled their forums on 2ch's, having observed the benefits of open posting.
Another one of the mentioned principles is "anonymity counters vanity". The idea is that registered users will become cliqueish, protecting their turf and jockeying over pride and identity. An open, more anonymous system essentially gives less reward to pride of identity, and posts are more topic-focused. Though this seems to counter another bit of web conventional wisdom, which is that reputation is an incentive for good behavior. The 2ch observation is that, at least, reputation is a mixed blessing; people will behave better when they want to protect their reputation, but they will also spend more time maintaining, promoting, and jockeying over that reputation. The cost/benefit tradeoff is probably different for different uses. On eBay, reputation is needed to provide for some trust when making costly exchanges; on a message forum, perhaps it's just a distraction.