Alright, a couple of things.
Twenty percent is the minimum acceptable tip for a server who has provided for your needs adequately and with a cheerful demeanor. Servers are paid about $3.50 per hour which is usually just about enough to pay the taxes on their tips. They are also usually required to pay between two and six percent of the total of your bill (sometimes higher if there is a lot of alcohol) regardless of the tip amount to the bartender, host, food expediter in the kitchen, and table bussers. If you feel the service was lacking, talk to the manager and tip appropriately, keeping in mind that if you came to the restaurant hungry and left full, the server should not have to pay part of your bill in the form of the above mentioned tip-outs unless there was some extremely egregious behavior (e.g. sleeping with your husband, calling your kid ugly, throwing a pie in your face).
Also, here are some unacceptable behaviors:
* bringing and using silly string in a restaurant (this happened tonight at my place of business!)
* talking on your cell phone while the server tries to wait on you and then being mad that the he or she didn't take your order quickly enough (happens all the time)
* ordering water with extra, extra lemon and then using all the sugar on the table to make lemonade in an effort to save the two bucks to pay for lemonade (yup, I've seen it many times)
* telling the server you are ready to order and then making him or her stand at your table while you read the menu (for pete's sake, are you the only person in the stinkin' restaurant who might need the server's attention?!)
* talking to your server as if he or she is an incompetent moron because of the line of work he or she has chosen
***** end of rant*****