Upvoting GOOD posts and comments should gain karma--that contributes to the community. Why not emphasize the positive, and discuss what "quality" of submissions and comments means?
I like to ask metaquestions that emphasize the positive from time to time, usually just after someone has complained about how HN is going to the dogs. Here are some examples, which I also have linked in my user profile:
I'm seriously interested in anyone's response about what the difference is between high-quality posts and comments and lower-quality posts and comments. If we can reach consensus on that, we'll all know how to use whatever voting and flagging powers we each have.
I think that the overall quality of HN posts has held up very well given the growth in the community size. OK - some of the stuff that I find interesting and which might have got a bigger feature a couple of years back can tend to drop out of view a bit quickly under posts about "pop" tech companies but they are still here and findable.
I am currently working on a "specialised" social bookmarking site and have gone with only up-votes (posts and comments)and a facility for flagging spam or the inappropriate. I am confident that this simple approach will share some of HN's benefits.