My concern with any change made is that it would encourage people to try and get a bunch of karma which could have unforeseen effects on the quality of discussion/submissions. I'm sure you'll be taking that into account for any change made, though.
Downvoting posts could be used to build algorithmic feeds, so users could see content corresponding to the ratio of items they voted up or down. But that might be too much of a "social media" feature and I could see Hacker News getting out the torches and pitchforks for something like that.
I think both in practice and in theory the function downvotes serve for comments are served by flags on posts.
I know, I know, that's an old one as well. But hey, it would be really really nice.