The kinds of things you can learn from focus groups are the things you need to learn early on, before you begin designing the site. Focus groups are for EARLY in the process. But don’t mistake them for usability testing. They won’t tell you whether people can actually use your site.
Don’t make me think
Being an expert in online usability Krug gathered his experience in a wonderful book. This summary of key thoughts will help you to remember most important principles and laws.