tflight - 9:45 pm on Feb 15, 2007 (gmt 0)

Usage factors may or may not be used, and if so there's no way it would be an important enough factor to trigger serious penalties or filtering.

Why not? Take a given search phrase, "Big Blue Widgets". Take the top 10 results. If 9 of the pages in the top 10 have a bounce rate of say 10% yet one of the other pages in the top 10 has a bounce rate of 60% then I (as a search engine) would really start to have my doubts about the page with such a high bounce rate.

If I'm trying to build the best search engine I can and the users are not finding what they want on that page then I (as a search engine) would not want to keep showing that page in the top of the results.

but penalizing pages because they are popular and sticky seems even too weird for them

I'm not suggesting that they are penalizing pages because they are popular. I'm sort of suggesting the opposite... that for whatever reason the pages are ranking well for a certain phrase but based on user data Google finds that people who search for that phrase and then click on that result end up quickly bouncing. To me that would imply that the page might rank well, but isn't popular with the users who end up going there.