I think that a lot of people don't realize why tipping is important. Your service staff has to 'tip out' a portion of their tips to other restaurant staff (ie. kitchen, bar, door, etc). The tip-out is based on sales, it is NOT based on the amount of tips they accumulate in a night. Therefore, if the server didn't receive any tips, they would have to tip-out from their own pockets - essentially having to pay to serve their tables.
Most service staff are young kids trying to pay their way through school, etc and depend on their tips to be able to pay their student loans. Depending on where you live, these kids make minimum wage at the very most. In some areas, servers are paid less than the standard minimum wage.
That said, North American tipping customs generally call for anywhere from 10-20% depending on the quality of service.