Ever visit a website you frequent and get frustrated that you can't find the log in button? For some reason that I can't comprehend, web designers love to highlight the "Sign Up" button and not the "Log In" button, communicating that potential users are more valuable than loyal, regular users. Here are some examples:
Sure, regular users are logged in most of the time they visit websites. But I can't think of a single usability reason why not to make log in just as apparent as sign up. It's a very mild inconvenience, but an avoidable one, so it shouldn't happen.
Facebook doesn't get a lot of praise nowadays, but this homepage is perfection. There is a clear way to log in, clear way to create an account, and no unnecessary content to distract the user. The more straightforward the web is, the better.