Watching Will and Diane's relationship crash and burn. You just have to cringe.
For that matter, watching the relationships between Alicia and Will, Cary and Diane, Cary/Alicia and the firm, and just about every other relationship on the show just explode one after another in the beginning of season five.
Diane breaking into tears when she realises her dream of becoming a judge probably just got squashed.
Will as he makes the decision tree in "The Decision Tree." The look on his face as he makes his set of questions for Alicia, and the resulting fantasy/flashback, is heartbreaking.