I find it lamentable that there are so many restaurant patrons who require a near-perfect dining experience to leave even 15%. What if the person who signed these patrons' paychecks decided, "You know, so-and-so has had a poor attitude lately...I think we should just give him half his pay." Wouldn't fly, would it?
Many of the things that could cause a poor dining experience are outside of a server's control--cold food, long waits, incorrect orders...these are rarely the fault of a good server. And if the server's attitude could be better, cut her some slack. You don't know how other patrons have been treating her that day, or what might be going on in her personal life.
And for those of you who regularly leave bad tips--you are only confirming the server's suspicions. They can sense it. Leaving a bad tip does not send a message that the server needs to improve, it sends the message that you are a bad customer.
If your experience was really that bad, don't go back to the restaurant. Or request a different server next time. Or let management know how your meal could have been better. These are far more effective ways of getting better service than leaving a lousy tip.
Bottom line--when you eat out, 15% gratuity should be the absolute minimum (and only if your poor experience is directly caused by the server). 20% should be standard. And for great experiences, any amount above that.