Also, pages that I left open for a real period of time would have their weights increased over ones that I closed quickly.

Refinement: detect "interest" by scrolling behavior/timing. Screen for "away from keyboard" behavior (which doesn't zero out ranking -- I may intend to come back to the piece later -- but should stop it accumulating value while there is no input for extended periods).

(I've been working on "Adaptive Training" for work, so this sort of "read the user's mind" tech has been much on my mind.)