A simple method might be to just build on the "avg. karma" calculation that HN already does, and use that as the value of your upvotes. Have an average karma score of 3? Your upvote adds three points to the score of the post.

This is based on the assumption that people whose own contributions are high-quality are also more likely to appreciate high-quality contributions posted by others. Using cumulative karma or seniority as a basis for vote weighting would make quantity - of posts or time - equivalent to quality, and might dilute things a bit.

The drawback here is that any system of vote-weighting, when coupled with using voting to indicate agreement or disagreement, might encourage a greater level of groupthink, and leave less room for discussion of controversial topics.