Birth year: They keep starting at the current year, when most people will be 18+ and have to scroll more than 18 years to get to their age. They feel old and have to scroll a lot.
Date Picker vs just typing it in: unless the date requires context of other dates or you need to select a range, just let people type in the date. Its proven that people don't mind typing in short fields. They aren't as dumb as you think. haha
Date Ranges: Allow for dragging.
Date Picker: Help your users out and include common holidays (maybe depending on country chosen) and integration of their own calendar. I knew a few times I made the mistake of choosing something over another date.
Birth Year: Just forget a picker for this, let me type in my age. (Edit: Pointy added that you may not want to do this when the year is important because some people view their age differently. To add to that I know that in Iran, for example, they count their age differently and so count themselves 1 year than those in the U.S. would.)
Happy Birthday: If they the enter their birthday into the form, and it's the day, have some special message for them.
Choose generic mark up: Don't give your forms special names be generic so autofill takes care the rest.
3rd Party Log in: When you are able, let them skip out on forms all together and sign in with twitter, facebook, google, etc.