Just read a post over at Bill Slawski's SEO by the SEA blog where he takes apart a recent Yahoo patent, and theorizes that:
How much might the usability of a web page matter to a search engine? If that search engine were to look at an approximation of the layout of a web page, it could try to understand how good of a user experience visiting that page might be, and evaluate the page based upon certain characteristics that it finds upon the page.
This is interesting, and while it is not implemented and could be years away from implementation, it brings an interesting, new, and fresh angle to look at what the search engines may be up to. For those people trying to stay just one step ahead of the search engines (which is not smart, duh). You should look at this as an idea for where the search engines may someday use to determine your spammy, low value, link heavy site isn't so great. Google might look at bounce rates for those who install Google analytics someday.
Of course for most people who just build quality sites with users in mind you wont' have to worry about this very much, that's of course the right way to go. However if these experiments ever become main parts of ranking algorithms, it will be good to know that these kinds of metrics could be used as areas to troubleshoot...someday. I say someday because we could be years away from these tactics being used by search engines to determine rankings.
Interesting to consider thought, right?