Therefore I suppose that lowest karma comments of high karma contributors would mostly be trivial snarky remarks downvoted for incompatibility of sense of humor. At least this is my hypothesis - let it be tested, if we can collect such comments somehow.

In response to Jack's expression of interest I went used Wei_Dai's script to download all my comments and had a search through with some regex. By approximate count the greatest number of downvoted comments were jests of the type you mentioned, followed by comments of the form "I don't approve of the grandparent either but your specific criticism Y is wrong for this logical reason". The lowest vote that I spotted was -6 for a comment along the lines of "I fundamentally disagree with your accusations of me and do not wish to continue this conversation".

The selection here is somewhat biased in as much as I am comfortable deleting comments if for any reason a conversation is unsatisfactory to me. There are quite possibly comments or jokes that would have gone into free-fall if I did not delete them when they reached -3 in 10 seconds flat. The downvoted comments that remain I either still endorse, consider important for the conversation to make sense, haven't noticed or don't care about enough to click on. (This isn't to say that deleting a comment indicates that I do not endorse it entirely. I also have no problem with choosing my battles.)

I'm afraid Jack would be disappointed in that few of the most downvoted comments seem to be about object level subject matter. Or, if they are, it is object level conversation about something that people are... passionate about. It isn't a source of ideas I have that people most disagree with, which may be interesting to see!