At the last less-wrong meet-up I attended, several people noted that they did not even comment for fear of getting negative karma on comments. I was one of only two people out of 12 with karma over 25, and some of these people are long-time readers. People who avoid posting based on the new rule might not be karma optimizing, but then you need to consider what's the purpose of karma to begin with-- is it to create a hierarchy based on score, or is it to discourage trolls and encourage useful comment? If it's the latter, I think the new rule will discourage more first-time posters than it will encourage, as 5 downvotes on a post will literally wipe away what might be a few month's worth of collected karma from shy commenters.