Without customer input and data from observations of users at work, it's easy to create laundry lists of user problems and feel like you have to solve all of them. Market research often results in such laundry lists. The problem is that these lists are often based on your company's preconceived understanding of what the problems are, and so can't tell you what you don't know about your users. Site visits also help you profile your customers in a much richer way than most market research, which provides statistics and outlines trends but often fails to account for the human side of the story or the reasons underlying the trend. It is necessary to look at real behavior to find out what people really do, not simply what they say they do.
Focus groups are close to worthless as a way to drive or evaluate design.
Listening to users can be misleading. For example, we recently completed a major competitive study of a group of B2B sites selling certain complicated equipment. Sure enough, another vendor, who is known as a low-quality provider in the real world, got low scores. But when we looked at what users actually did on the sites, they were much more successful shopping on the site from the vendor they didn't like. The site from the popular vendor provided very poor support for the shopping process.