Most of the other responses deal with changes to the karma system - I'm not sure that's the only way to go. How about placing some nice text above the comment

Most of the other responses deal with changes to the karma system - I'm not sure that's the only way to go. How about placing some nice text above the comment submission field that reminds the poster to be nice, ie "Please be polite.", or even better, throw up random literary quotes about politeness/helpfulness/communities. It would be a neat experiment to see if the ratios of mean-spirited responses to other types of comments decreased due to the suggested (mine or otherwise) changes.