I've started a list of the ones I think we should all agree on—a set of UI Rules, if you will. First on the list is one of the easiest to fix.
Rule #1. Focus on the first text field.
Most important for login and signup pages, but this rule applies to almost any form on any page (with the possible exception of pages with lots of separate forms). Why not save your users a click when they're performing the primary action on a page?
Let's look at some examples. I don't mean to pick on any of these companies—my methodology here was picking randomly off the list of trending startups on AngelList.
Assembly does a better job—when you click "Login" on their page, the email address box has focus. They fail in a more subtle way, however—when I click "Signup," it takes another click to set focus on the Username field. (Perhaps they expect me to click on the Facebook signup option).
Let's all agree to follow this rule from now on—however you do it, focus on the first text field on the page. In almost every case, you're saving your users time, which will lead to increased engagement and conversion rates. Together, we can make the Web a less frustrating place.