On the other side of the coin.... My brother once refused to tip a waiter who gave him horrendous service. Ignored him all night, bad attitude, slammed cold food down in front of him, took forever on his bill. So my brother's a good guy, he works in the service industry. He refused to tip him and the waiter finally woke up, came marching out of the restaurant and basically threatened to fight him in the street over the lack of a tip.
I take a softer approach to basically what Mr Pink said in 'Reservior Dogs.' A tip shouldn't be automatically expected. You do good work, you're fast, you're friendly, then you'll get a good tip. Hell, you might get a very good tip. I've given 40% tips in the past. But why should I be forced to tip someone who makes my dining experience utter ****?