On one level it could be a simple bookmarket (bookmark with JS). When clicked, it goes through the HN discussion page. When it finds a top HNer's username, it (1) colors it and (2) shows the category if that HNer is in the top or bottom 10% in any category.

For instance: edw519 33 minutes ago | link Top: Cheerfulness, Orderliness, Gregariousness, Agreeableness Bottom: Imagination, Authority-challenging, Intellect

(I'd also something that shows me when a commenter is somebody important, even if not a karma king.)