This question might be too general but its bugging me a lot.
If you look at the screenshot below:
Maybe J have been sending emails for too long, but I expected the 'CC' and 'BCC' fields to be visible to me. The Gmail team apparently thinks these fields need not be shown unless someone clicks on the 'to' field, as below:
I found this to be distracting and irritating. When sending an email, the CC field is one of my most used and most important tools, and it's hidden by default!
I know its purely contextual, but in terms of a guiding principle, what control from a UI is it acceptable to hide by default?