Personally, I tend to want to respond to an article or discussion with whatever thought I'm having at the moment. The HN karma (in particular, the average) reminds me to try to filter my comments based on whether I'm saying something that I think others will find useful or whether I'm just saying it because I feel like saying it. I probably censor about half of what I would otherwise post based on this guideline, and it's nearly always to the betterment of my signal/noise ratio.
I find myself willing to spend more time/thought crafting a response to an article which I think will be read by more people. For example, I'm just pounding this out stream-of-consciousness style since I know it will only be read by those involved in this subthread, if that. Why not do the right thing and focus on thoughtful, well-written comments for their own sake? Because I have the karma carrot dangling in front of me. I feel like gamification has in this case removed my motivation to do what is most beneficial to the site.
