I think Less Wrong has an abnormally high percentage of lurkers because if participating at any web site is intimidating, participating at Less Wrong is especially intimidating because of the high level of discourse and English linguistic proficiency.
For the strictest definition of lurker, if you have registered for an account you are not or are not longer a lurker, but the definition is really not important.
This definition of lurker has the advantage of being clear-cut enough that numbers are meaningful, but does not represent as important a group in online community dynamics as the definition of lurker as someone who reads but does not post, regardless of whether or not he has an account.
Also, with that definition, I have not been a lurker for quite a while, and yet I appear to be accumulating free karma points for saying "hi" anyway. Not complaining.
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.