The ability for each user to create their own personalized domain blacklist.
E.g., if someone doesn't like techcrunch.com, they can add it to their blacklist, and they will no longer see techcrunch.com stories.
There's maybe a dozen sites (TC isn't one of them) which regularly appear on HN, and which I'd be happy not to see. But I know others don't feel this way about those sites. This seems like a relatively easy way of improving everyone's experience.