I tip at WDW the same as I tip at home - 10% for buffets, 15% for sit downs. I add more if the service was above average and have been known to leave 20% at buffets when the waiter went truly above and beyond. But since i calculate the tip based on the total, including tax, the real percentage is actually a bit more, and I always round up to the next dollar.
I think it's shameful that the law allows restaurants to pay waitstaff less than minimum wage. I also think it's shameful that people in the US automatically expect a tip no matter how well or poorly they do their jobs. I paid for the meal, and the waitstaff's wages come out of that money, so to me a tip is something you give someone who has done a good job. It's a gift of appreciation, it's not supposed to be part of their salary.
And I have no problem in leaving a much smaller tip if the service is bad. I have been known to leave pennies or nickels on the table just to let the server know that I didn't forget the tip, I just thought they didn't deserve one.
The best way to get a good tip from me is to make sure i never see the bottom of my Coke glass. The longer my glass goes empty, the smaller a tip you get, but if you keep filling it before it goes more than 75% empty, the tip will be larger. I also count attitude and manner toward tips; if you're tired, okay, but if you're snippy, short, or especially if you're outright rude, don't count on seeing many pesos on the table after I leave.
If you make a mistake with my order but correct it right away and appologize, the tip will still be good; after all, nobody's perfect and I don't expect them to be. But if you fail to appologize, that's not right, and the tip will be a little smaller. And if I have to ask over and over to get the mistake corrected, look for a goosegg.