David Letterman retires: Here's the most important thing that happened each year of his career

How do you measure the impact of someone who changed late-night television? Simply put, you can't. But you can chart the most important thing that happened to him every year of his career. Just before David Letterman signs off "Late Show" for good on Wednesday, here are the most significant highlights (and a couple lowlights) of the host's 33 years on late-night TV.