There have been a lot of comments stating why this is a poor idea, but as someone who is sub-1000 karma, I haven't seen this reason posted yet (although I could only read the first half or so). In a typical day, I can't spend all day sitting on Hacker News refreshing threads and seeing if anyone has responded to me (or promoted my comment). But occasionally I'll read a thread, find a couple of interesting places to comment, and then go about my day until I can check in again later. Only being able to leave one comment in a single reasonable-length session would absolutely kill that use case. And frankly, it would make this site much less attractive to browse as an occasional commenter. Maybe you're okay with that, but I think that would be a mistake.