If you use 'Tab' to quickly move between form fields then I'd say that its now muscle-memory to 'type username. press tab. type password. press enter'. Well yahoo manages to break it, by introducing a linked email id, right between them. which means i end up typing my password twice. Its 2008 people! If you don't know what to type in the username box, you shouldn't be using it.
A big part of creating a good user experience is choosing the right defaults. A calendar is a particularly striking example. When you use a calendar widget to fill in a date, the context should determine a reasonable default date. If you were booking a plane ticket, today's date would be a nice default. But if you are entering your birth date, today is so NOT a good default, but it is seen here