A recent post by Matthew Yglesias makes mention of a common casualty of poor usability on the web.
Every day, users abandon websites when they cannot figure out how to do what they came for in a reasonable amount of time. We don’t know how long Matthew spent trying to find the information that he was looking for, but we do know (from a post by Jakob Nielsen that the average page visit lasts about 30 seconds, and more experienced users spend even less time. Most website visits last about 2-4 minutes. When users can’t accomplish the task they came for within that period of time, they will typically give up. Poor usability can prevent even a powerful website from being useful for it’s intended purpose.
Brendon Robinson's Blog 