What really grinds my gears is when people don't tip, for whatever reason. Regardless of circumstances, there's no excuse not to tip two dollars at the very least. If you can't afford to tip two dollars, you probably shouldn't be ordering pizza in the first place. Three dollars would actually be better, because that would be just enough to pay for the cost of gas to your house and back.
But then, we live in a society where the customer is king, even when they're wrong (which is quite often), and the employees just have to bend over and take it.