Focus groups can be great for determining what your audience wants and likes - in the abstract. They’re good for testing whether the idea makes sense and proposition is attractive; to find out how people feel about your competitors. But they’re not good for learning about whether your site works and how to improve it.
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.