Also, the karma system adds an additional barrier, at least in my mind. Knowing that your comment is going to be explicitly judged and your score added to a "permanent record" can be intimidating.
Whether we like it or not, that "intimidation" may be the single most important factor in keeping the level of discourse in the comments unusually high. Status games can be beneficial.
Some research says that lurkers make up over 90% of online groups. I suspect that Less Wrong has an even higher percentage of lurkers than other online communities.
Please post a comment in this thread saying "Hi." You can say more if you want, but just posting "Hi" is good for a guaranteed free point of karma.
Also see the introduction thread.