Design
The graphical elements and layout of website have a strong influence on how easily usable the site is. Standards like blue, underlined links, top and side menu bars, logos in the top, left-hand corner may seem like rules that can be bent, but adherence to these elements (with which web users are already familiar) will help to make a site usable. Design also encompasses important topics like visibility & contrast, affecting how easy it is for users to interest the text and image elements of the site. Separation of unique sections like navigation, advertising, content, search bars, etc. is also critical, as users follow design cues to help them understand a page’s content. A final consideration would also take into account the importance of ensuring that critical elements in a site’s design (like menus, logos, colors, and layout) were used consistently throughout the site.
Information Architecture
The organizational hierarchy of a site can also strongly affect usability. Topics and categorization impact the ease with which a user can find the information they need on your site. While an intuitive, intelligently designed structure will seamlessly guide the user to their goals, a complex, obfuscated hierarchy can make finding information on a site disturbingly frustrating.
Navigation
A navigation system that guides users easily through both top-level and deep pages and makes a high percentage of the site easily accessible is critical to good usability. Since navigation is one of a website’s primary functions, provide users with obvious navigation systems: breadcrumbs, alt tags for image links, and well-written anchor text that clearly describes what the user will get if he or she clicks a link. Navigation standards like these can drastically improve usability performance.
Functionality
To create compelling usability, ensure that tools, scripts, images, links, etc. all function as they are intended and don’t provide errors to non-standard browsers, alternative operating systems, or uninformed users (who often don’t know what/where to click).
Accessibility
Accessibility refers primarily to the technical ability of users to access and move through your site, as well as the ability of the site to serve disabled or impaired users. For SEO purposes, the most important aspects are limiting code errors to a minimum and fixing broken links, making sure that content is accessible and visible in all browsers and without special actions.
Content
The usability of content itself is often overlooked, but its importance cannot be overstated. The descriptive nature of headlines, the accuracy of information and the quality of content all factor highly into a site’s likelihood to retain visitors and gain links.