I wanted to wait until today to write my debut post because I didn't want to steal the thunder away from Barry's reporting from the Chicago SEO conference. Those of us who couldn't attend hang on every word you know! (Especially with what's going on with Google these days.)
Usability is many things. For the purposes here, I'll focus on website usability and help you find ways to make your web pages more user friendly. Many times the same logic applies to both humans, search engine robots and directory web page reviewers. Each wants to learn what a web page is about and they want this information quickly.
Sometimes it's a simple matter of being able to read a page. There's some new CSS tricks that allow your visitors to change the font face or font size of the web page to make it easier to read. If you're interested in Typography, The new typography by Clagnut has a good writeup on CSS, font families and their own "Switch Typeface widget".
What's cool about this article are the references to Linux fonts - not just Microsoft.
Don't know what fonts are on your system, or the standard in other computers? Try VisiBone Automated Font Survey
Content is often unreadable because it's been color coded and placed against a colored background. There's not enough contrast between them, making it quite difficult for your content to be read. How silly is it to write a product description your potential online customer can't see? I have a lot of color tools that tell me quickly what colors go together well, and what their HEX or RGB code is. Here's a new one, called the 4096 Color Wheel
Of course, website usability covers way more than fonts and colors. But, you'd be amazed at how many people struggle with websites because of these two basic things.
If your target market has eyes, consider what they need to use your website with ease and add these requirements to your website's design objectives.