One simple change would be to replace time-based "gravity" dragging items down as they age. Instead, gravity could be based on the number of users who have "finished" consuming the content. Perhaps by just clicking "Back" to the aggregator, or by explicitly acknowledging the completion. There could be three buttons: Like, Dislike, and Saw But Don't Care. In the algorithm to drive items downward, time would be replaced by the count of all three actions. Long-form content would then not be penalized so much (but would still be disadvantaged by users who dislike or don't care about long-form content, of course).

A New page might also need to be sorted differently, e.g. by putting items with zero count at the top, and ignoring time as with the front page.