I agree that it has improved the civility of discourse.
Have you considered allowing users to turn karma display back on once they have reached a set amount of karma, much like other features (such as downvoting)? I imagine that, as long as the threshold is high enough, users would have learned how to be a benefit to the community by that point and, in addition, it might remove some of the psychological tendency to "score points off of someone" if only a subset of users can see those points.
I found myself to be fairly susceptible to reactions based on other people's karma scores; I don't see why it's a good idea to encourage an "elite" of HN users to write dumb comments.
It would be pretty easy to add a feature such that once you've seen the karma scores of comments in an article, you can no longer comment on them. That seems like a fair trade to me.
What about alt accounts? Sure, you can try to track that via IP, but by the time you start thinking about that, even this "pretty easy" feature starts being real complex and prone to problems (like misidentification behind NATs, etc).
I, too, found myself susceptible to reactions based on karma score and, personally, it wouldn't bother me at all if the scores never come back. If given the ability to view karma scores, I would leave them off, since I'm self-aware enough to realize that -- when karma scores are enabled -- my comments are of a lower caliber.
However, I'm not against allowing users with a certain level of karma the choice to view karma scores on threads if they really feel that HN is better with karma on, which it seems like a fairly large subset of users (or at least a significantly vocal one) wish for. I think that StackOverflow's successful moderation policies (and, to a lesser extent, HNs own) have sufficiently demonstrated that extraordinary members of a community can handle the responsibilities that come with increased privilege.
For example, I suspect that, if pg allowed individuals with high karma, such as yourself, to choose to toggle an option such as "view karma scores", no flood of dumb comments from high karma individuals would suddenly manifest. Further, I think that the transition from default karma displayed to our current situation has allowed anyone who has invested significant time in the community an opportunity for introspection that wasn't previously available. I'm now able to evaluate, personally, how my comments and behavior were effected by the display of karma and I'm less susceptible to the psychological effects as a result.
Of course, even if there isn't a significant negative aspect to allowing high karma individuals to view karma scores, it's still hardly worthwhile unless there's some kind of positive impact as a result. Perhaps mollifying the community would be enough of a benefit? Maybe a more pro show-karma individual would be better qualified to comment on the potential upside. My feeling is that it's possible that some subset of HNers exist whose experience with the community is improved by having karma scores enabled and, as such, it would be beneficial to allow them the possibility of a choice.
Have you considered allowing users to turn karma display back on once they have reached a set amount of karma, much like other features (such as downvoting)? I imagine that, as long as the threshold is high enough, users would have learned how to be a benefit to the community by that point and, in addition, it might remove some of the psychological tendency to "score points off of someone" if only a subset of users can see those points.