For six seasons, we've seen Don blow deadlines, alienate or fire clients, hobo out to California on a whim and spend entire days in movie theaters and bars. He chews out underlings -- the ones he actually notices -- in a cavalier fashion and he often treats his partners as annoying encumbrances.
Sure, he's often brilliant enough to make his pain-in-the-ass qualities bearable, but he's erratic as hell. He's been known to intimidate colleagues and even family members into putting up with his worst antics. He has a memorable glare.