T.R. Knight didn't get all that much to do during his last season on Grey's, but George's death broke our hearts into smithereens. Just as he's about to leave Seattle Grace to take up a position in the US Army, George gets hit by a bus. Since he's disfigured, the doctors admit him as John Doe, only to later realise his true identity when he traces "007" into Meredith's hand. A few beats later, he's brain-dead.