I'm a huge lurker on many, many sites, but I probably lurk the hardest on HN, Reddit and Hubski. Fear of being wrong, and a lack of respect for one's own opinion I'll admit are big factors, but I think there's also an issue of being late to the party. I see threads on Reddit's 'Hot' feed, that have been weeded out by the Reddit hive-mind as being full of good discussion and content. But as soon as I get to the comments I suddenly get extremely disappointed because there's been a discussion raging for the past day or so. I, personally, feel that often a lot of what needs to be said, has been said. On the flip-side, when I do have something to say, often I feel that there will be very little audience to see it and to respond to it. It's a bit like, 'what's the point' (I don't feel this applies to Hubski, which puts new comments up the top)
Hubski really solves a lot of this problem by allowing people with a large numbers of followers to share threads and links posted by an occasional lurker that comes out of the woodwork. However, I think it's intimidating for some new users to see people with a huge amount of followers and make huge contributions to the Hubski community. Maybe a way to search and subscribe to new users that have certain interests? To join a new website and be quickly greeted by a "### has started following you" would really really encourage new people to feel like they have an audience that they feel like they can connect with.
Just my two cents.