This one, I thought needed no explanation but after working in restaurants you see just how cheap people can be when the bill arrives. Tipping is the only way your waiter, waitress, or bartenders make a living. They receive an hourly rate below $3 and that just pay some taxes. The starting percentage should be 20% or higher depending on how satisfied you are with your service. If however the service is substandard or just plain terrible then move the percentage down to as low as you see fit.
I have seen that there are certain types of people that consistently tip poorly and I don't think they are aware of just how dreaded they are by this entire industry. Some terrible tippers even have the nerve to keep visiting the same restaurant over and over again. I'm not saying that someone should mess with there food although I'm sure it happens (I would never do that because I am very particular about how food is treated) but they shouldn't be allowed to eat at the establishment.
Let's be clear here 15% is not acceptable. In the 80's and maybe the early 90's this worked but now that's just not enough. I'm sure if you've never been a waiter or a bartender this probably makes less sense to you. Most people think waiters have a pretty easy job; they take your order and bring out food but most don't know that everything besides the kitchen (even part of this) is cleaned by a waiter or bartender. Every piece of silverware is rolled and cared for by the wait staff, and they usually even have partial responsibility when it comes to cooking duties such as salads, desserts and some appetizers. Plus 3-5% of the tip goes to tipping out a bus boy/girl, hostess, and any other auxiliary person used. Leaving 10-15% consistently is wrong and says that you don't care if that person serving you can pay his/her rent or feed his/her child.
How we treat those who serve us is a reflection on how we treat others.