Leah's asking the right questions, but I don't think the direction she took is the right one. Users rely on the UI to be familiar, and I've always found Amazon's UI strange in that I have to pause, analyze, and think when logging in/signing up. While Amazon might be able to get away with it, removing the familiarity from the UI is probably a bad decision for most. If an extra field is a barrier to sign-up, the user actually having to actively analyze and parse your sign-up form is a huge mental hoop to jump through.<p>We (Weebly) thought of this problem early on, and we tried to simplify things by just requiring an email/password when signing up. What we found was that our users <i>were extremely confused</i>, because the sign-up form didn't look like a sign-up form. When doing usability testing, we found that users would stare at the page for minutes. "us: Just sign up..." "user: I can't see where to sign up", even when it was right in front of them. They saw two fields and assumed it was a log-in form, even when it said in really big letters "Create a new account".<p>Users don't read your page, they scan it. People expect the log-in form to be in the upper right and the sign-up form to be towards the center of the page. You should absolutely make your site scan-friendly.<p>Users do, though, get confused and try to use the sign-up form to login. Our solution? If the user enters credentials into the sign-up form that already exist (like the correct username/password combo), we'll just log them in instead of creating a new user. You can try it out on weebly.com.