Mine is always think about user intent. Why are people coming to your site in the first place, and why should they click on ads when they're there?
That is a really, really good point.
I wonder though how we are going to keep up with figuring out user intent now that the search terms used to enter our site are more obscured by google - and will probably continue to be more obscured as more people eventually get google profile / gmail accounts.
I have used my internal search logs (ones that log queries when someone searches for something on our site), and unfortunately the queries are so vague and there are so few of them) that it is really hard to draw a conclusion about what their intent is.