one warning about how some sites implement the month numbers/month namespace which drives me up the wall: make sure that what people see in the drop-down list is actually how it sorts. For example, sometimes the displayed text is "January" but the text when you actually get into the drop-down begins "01-January" -- which is much more difficult from a keyboard access perspective!

Other pet peeve: when the US states are displayed with their full names but sorted alphabetically by postal abbreviation, or vice versa. This happens more often than you would think.