Smashing Magazine has an article that shows common website usability problems and ways to avoid or fix them.

“The goal of user interface design is to make a website easier for visitors to use. For online stores, fixing problems brings in more visitors who browse more products for a longer period of time. Even simple fixes, like not requiring registration for purchases, can have a huge effect on your bottom line. Keep these common problems in mind the next time you design an online store. Your customers will thank you for it.”

Having visited a large number of sites in my time on the Internet, I have to agree with the observations as well as the remedies.