Don't even get me started here.... A tip is exactly that. It is something that is given for GOOD or even EXCEPTIONAL service. I am sorry that servers do not make very much money. But, they knew what the job payed when they took it and they also know what to do to make extra money. It is understandable if the restaurant is short staffed or over booked or the problem with poor service arises from another area like the kitchen; persay. In this situation of course a tip is still deserved. The poor server has no fault in these events. With good service the standard 15% tip, I believe, is acceptable. With excellent service then I believe that the customer should do his or her part to show their appreciation, above the 15% amount.
In other areas like bellhop or doorman, I believe that the situation and area of the country dictates the amount of the tip. Again, more for excellent service with large baggage or the hailing of a taxi expediently. I believe that this tip should be no less than $5.00, per event or more based on service.