Store owners who know too much about their products. :) When you know all of the right industry acronyms and phrases, it's difficult to transfer your intimate knowledge to the web where the audience won't have your insight. You have to think like your customer and present your product as a solution to their problem. You have to forget about what YOU would type into a search box and try to think of what a customer who didn't know what you know would type. If you get this one right, you'll make money all day long.