For me it was when prime raised it's price to $125, they stopped giving me any credit when my prime deliveries were late (they were more often than they weren't), and so I decided to walk over to Target in my neighborhood and see if it was more expensive -- Target was cheaper.

At that point I decided the experience of going for a walk when I need something in a pinch was nicer than ordering online, and if I need something obscure, I'll pay for shipping once or twice a year.