I rarely think about karma before posting, but I used to think about it a lot more. It was never about following the HN group think, though. There is some of that here, but it really isn't comparatively that bad. In general, it just makes me think twice before posting a comment that isn't well thought out, adds nothing to the discussion, or is actually outside of my knowledge. It also encourages people to simply post more respectfully.
As I said though, it doesn't really matter much to me anymore.
It would be a fun experiment if pg turns off for a while displaying the current karma of comments and/or posts. Ranking would still give some hint what is popular, but that would be much more subtle and I think groupthink would be discouraged as a result.
Even funnier, he can split the users in two random groups (one seeing the karma, the other not) and actually measure how groupthink affects voting.
As I said though, it doesn't really matter much to me anymore.