EDIT: The below is based on my misunderstanding of terminology. It turns out "retract" and "delete" are different things. Oops.
Huh. To me, that sounds like exactly sort of behavior a karma system is supposed to encourage. Post worthless comment => comment downvoted => retract comment => future readers don't waste time reading it.
I retract [EDIT: delete] comments in high-traffic subthreads threads if they don't get upvoted after a day or two. Should I stop doing this?
(Disclaimer: I did not see the original before it was deleted.)
I delete comments in high-traffic threads if they don't get upvoted after a day or two.
You don't need to do that. The voting system already allows good comments to rise to the top, and because it gives a bonus for recent ones you don't need to worry about your old unvotedup comments hiding new promising ones.
http://www.quickmeme.com/Overcoming-bias-guy/popular/1/?upcoming