It bothered me that I was never able to easily remove an incorrectly typed address from Mail's cached list of addresses. For instance, if I mistakenly emailed a friend firstname.lastname@example.org, but his address was really email@example.com, the .com address would always be the first to show up in the address bar when I would begin to type a message to this person. As of 10.3, you now have the ability to edit or remove any address that Mail has cached.
Just start typing an address for a person, and let Mail auto-complete it (press Tab once it has the right person's name selected). Then, click and hold (or Control-click) on the name in the "To" field, and you are presented with a drop-down where you can choose to edit, remove, or open the person's entry in the Address Book. Very handy.