There is a very interesting and telling story behind why Google displays only 10 search results by default. Well known Googler Marissa Mayer, the company's Vice-President of Search Product and User Experience, discussed the logic behind their decision to only display 10 results.  They ran two tests – one with 10 results and one with 30 results, and found that traffic and revenue dropped by 20% with 30 results because of a 0.5 second delay in displaying results.  That small load difference made Google less usable and desirable for users.

It is those details that companies need to be aware of when building, designing, or tweaking their websites.

Color, speed, appearance, layout, menus, links, and even fonts can dramatically affect the traffic and conversion rate of a website.  There are many factors to consider when thinking about website usability for the customer, but here are some of the most important ones:

Color – Color evokes strong emotions.  Blue is the most common color used in web design, and for many good reasons – it is associated with calm, it meshes well with other colors, and it doesn’t strain the eyes as bad as, say, bright red.  Test out color schemes before implementing.

Typography – Beyond making sure spelling is perfect, make sure the language is clear, consistent, and to the point.  Professional presentation matters.

Purpose – If the purpose of the website isn’t explained or available to the user within several seconds of landing on any page of the website, then there is a problem.

Validation - The website should be validated for CSS, HTML, and Javascript.  This is important for not only usability, but search engines.

Speed – Half a second caused a 20% drop in revenue for Google.  Take out unnecessary images, manage website real-estate, and make sure the website’s servers are managed by professionals with a superior track record.

Navigation – The most relevant and important content needs to be accessible, categorized, and easy to click for the user.  Too many choices can risk the user being overwhelmed, while too few choices can risk the user not finding what he or she wants.  The best way to find the right balance is to constantly test.

Browser Compatibility – Internet Explorer 6 is reviled among programmers and designers because it breaks so many websites, but for almost all businesses, being compatible with the browser is vital because so many customers still use it.  Make sure the website works in all browsers.

International Considerations – While red is associated with anger and passion in Western culture , red is associated with good luck in China and death in Turkey.  Know the perceptions of the audience in question.  Put in the research to understand how international users may react.

These are only a few of the most important considerations when thinking about website usability.  Something as simple as the location of a shopping cart can double or halve revenue.  Constant testing is a must for understanding user behavior.

For more in-depth and technical usability checklists, check out the MIT Information Services and Technology usability guidelines and the Keynote NetMechanic usability checklist.