5 Factors of Usability:

• Learnability. How quickly can a new visitor learn the user interface to accomplish basic tasks? Is the design of the site intuitive?
• Efficiency. How quickly can users perform tasks, find products etc?
• Memorability. Can a returning visitor remember how to effectively use your site or application or are they forced to relearn everything from the beginning?
• Errors & Error Frequency. How often does a visitor make errors while using your website? How serious are these errors? Why are they making them and can they easily recover from these mistakes?
• Satisfaction. After completing their task does the user have a good feeling about your company?

Focusing on these five points will give you a good start on fixing basic usability issues.