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I have lately started to divorce "usability" from "accessibility." Thanks to things like AJAX, they are now no longer synonymous. I consider usability to be my #1 goal, but accessibility is immediately behind it.
Building content and getting links are all very well but if you're site is usable and makes more people stick around and navigate through - if google does take user behaviour into account then making your site more usable could be great for your optimisation..
What about accessibilty? Without meaning to be too controversial - do a lot of webmasters make their sites accessible to cater for everyone, or are there any peripheral benefits to it as well? (rankings perhaps?!)