I think the quality of Hacker News is sort of like Escher's "Ascending and Descending" (http://en.wikipedia.org/wiki/Ascending_and_Descending).

People perpetually talk about how the quality is going down, when in reality it's not changing much at all.

I attribute it to the "honeymoon effect". New users are excited when they first find HN, but eventually the amazement wears off, even though the site hasn't changed.