One of the first requirements for anyone considering building or operating a website should be Steve Krug's miraculous book - Don't Make Me Think. This short, 3-hour read has been next to my desk since I bought it and I doubt it will ever move. Mr. Krug's simple ingenuity and intuitive reasoning are why I work so hard to design with usability in mind.
A good thread at HighRankings discusses Mr. Krug's efforts and his recent interview with Management Consulting News, which sadly was not representative of his extraordinary insight and perception - either that or he's a much better writer then he is an interviewee. Also, if you have the budget, consider hiring Krug; I can say with certainty it will probably be the best website investment you make.
I was thinking to myself that usability isn't related to SEO and then I stopped myself and realized that it is important in the sense that it would create more happy users, more links, and better content, which IS related to SEO. So this is a great issue to discuss on an SEO sight. Well done.