What if we forgot the assertion that anyone has any idea what Google really wants, and just focus on whatson's question: what can we do to make visitors explore our sites more? There's no doubt that would be a good thing to achieve, and even if it doesn't rocket you to #1 with Google, it should bring you more quality traffic (unless you've made your site entirely dependent on Google traffic).
Onto whatson's question: there's been speculation that google must just different niches by different standards. I.E., wikianswers should have be bouncing off after five seconds because it's a quick answer site. Of course, you also bounce off if you get to the page and no one's attempted to answer your question yet, so how Google can tell successful Wikianswer queries from unsuccessful ones, I don't know. Any thoughts?
Most websites however CAN be judged at least partially by time spent by visitors on site. What can we do to keep them around? Besides "writing good content" - I mean, we all know that, but it's like saying "To become a successful artist, you should paint better." I think we can get more specific than that.