I'm someone who doesn't join in threads pretty often; I'll chime in if I find the topic to be something I'm interested in but my ignorance to most other matters leaves me from wanting to join into threads because of a fear of people piling on negative responses or "schooling" me in terms of the topic at hand.
I don't submit new articles because most things I'm interested in are discovered by more well connected individuals so I'm usually late the party so my score is relatively low despite how long I've lurked.
I'm fearful this new system discourages my participation further; if I don't add some insightful comment or my ignorance of a topic causes others to question whether my opinion is worth discussing I'll be kept from participating because I haven't built up a score. I won't even have the chance to join future conversations because my comments will be pending so until someone decides my opinion has merit I'll be censored from joining into other aspects of an article.
I may not be the most social of the HN folks here on the board but I do like to join into conversations when the topic is of interest to me. I guess this sort of system just makes me feel unwelcome because I'm being punished for not joining in earlier.
Even the most negative comments incite conversation; a person who may have an unwelcome opinion or whose ignorance prevents them from understanding a topic can learn from discussions based off their response.
I understand the reasoning for this and I think, for the most part, it'll help keep the comments section fairly civil. I'm just worried by solving this issue you're throwing away a part of the community who haven't had a chance to prove ourselves over time. I'm regretful I haven't spent time building up my karma in retrospect.