I've never been a waiter/waitress. I have, however, worked on a kitchen staff. During those two years I saw busboys, hostesses, and servers put up with a lot of bullshit - all while getting paid just half of the minimum wage (the minimum wage, not the livable wage).
Let's say I fuck up an order; the waitress involved will take 100% of the heat (from the customer), and her tip will suffer because of my performance. What is that bs?
Consider this: For a single person with no children in Vermont (my state) the livable wage (avg between rural/urban wages) is 12.18 an hour. I personally know waiters/waitresses making five dollars an hour. These people are absolutely dependent on tips to make up this huge divide. Of course, even then that is too ideal - rarely are waiters/waitresses in such good straits that they only require the 1p0c wage, when in fact many have children/are in school/in debt.
As long as the law allows for these people to be screwed, you'd better tip well. It's part of dining experience - when a person enters a restaurant in the United States, they understand (or should, anyway) that a tip for their waitperson is going to be required.
For the people complaining about the lack of effort involved in waiting tables ("All they did was set my food down! All I got was a beer! etc"), consider this: although your order may have been manageable, your party may have been small, and you may have been content and courteous, a waitress still has to wait on you, provide you service, while worrying about other tables and larger parties. You are paying for access to the waitress and attention, in a way - for those of you pulling the whole "but not me! i'm a great customer!", just understand that waiters (of course) are people too, and as such they are governed by the same economic mindset. Do you think a waiter really wants to hand you your house salad and iced water while he/she could be schmoozing with that table of eight? No. But (and this is based off anecdotal evidence), the vast majority of waitpeople still treat your table as well as a table that might offer a higher yield.
Just don't be dicks. It's not hard. I know that in my two years in the restaurant business, I always went above and beyond for good customers - so did our restaurant's waitstaff.