I'm thinking of classifying entries as temporal/atemporal. Temporal are industry news, press releases, rumors and alike. Atemporal are reference materials, essays, howtos, analysis, links to libraries/frameworks/products.
Essentially, I feel that while temporal entries provide a more immediate reward and larger discussions, atemporal tend to be more technical, more interesting, have more valuable discussions, and overall are what sets HN apart of other places. I'd rather see a front page full of erlang stories than pieces about what Apple has just announced (I'm pretty sure I'll see these in many different places).
A good balance is probably ideal: a few most interesting breaking stories, and the most thought-provoking atemporal materials users have ran into lately. The perceived decline of the front page and comment quality could be attributed to a shift in this balance towards temporal stories.
As to how to classify, I'd start as marking stories with multiple duplicates in a short period of time as temporal, and the ones with duplicates spread apart over long periods as atemporal, and go from there.