The largest part people do on the Web is to look for the next thing to click, it’s important to make it obvious what’s clickable and what’s not. When you force users to think about something that should be mindless, you’re squandering the limited reservoir of patience and goodwill that each user brings to a new 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.