Americans* who don't tip at least 15% for good service from those in service industries with well-established gratuity-based compensation systems** are scumbags. To me, people who pay less than 15% for a tip for decent service (I'll sometimes tip a server 25-30% if he/she gave service so good it made the meal better) are essentially committing a theft of the service, no different than if they tossed a $20 bill on the counter of a local liquor store and walked out with a $23 bottle of wine.
Bad service deserves commensurate tips. A couple of times I've had a server completely screw up a meal without trying to make things right, cop an attitude with me about inexusable delays or bad food, and they ended up with no tip or maybe a quarter. People who depend on tips but can't understand the need to give good service are imbeciles. Some friends of mine and I have been regulars for several years at a bar and I used to tip one of the bartenders there 30-50% even though she had kind of an attitude and gave short pours on draft beers. After we found out that for years she deliberately failed to tell us about a frequent customer program they had (at a cost of many free beers and meals that we missed), she discovered that I showed my gratitude for that and her short pours by leaving $3.75 instead of $5 for a $3.50 beer.
* Most other parts of the world pay their waitstaff and other service employees something closer to a living wage so they are less dependent upon tips and customarily receive less than the 15-20% tip that is standard in America. This post thus discusses tipping in the U.S., not elsewhere.
** However, I really get annoyed at the attempts over the past 15 years or so to expand the gratuity-based service provider pool into jobs outside of the traditional fields (servers, bartenders, barbers/stylists, bell hops, cab drivers, caddies, etc...). I had a snotty kid at a sub shop once clear his throat and very obviously nod over toward a jar that had a sign on it that said "Tipping is not a city in China." I was amazed by the gall this kid showed, so I said to him "I'll give you a $500 tip if you can tell me how you do anything differently than any other fast food worker and you can look me in the eye and honestly tell me that you've ever tipped someone at McDonalds or Taco Bell." He didn't say anything but judging by his glare, if my food wasn't already wrapped up, he would have spat on it for sure. I seldom frequent Starbucks or any other coffee shop, in part because I don't happen to care for the attitudes of a lot of the people who work at such places that they are more sophisticated, special, superior, etc... compared to their customers. Pal, you can call yourself a Barrista, the King of Siam, or anything else you want, but a) you're just somebody who does an unskilled job working in a coffee shop; and b) you don't deserve a penny of a tip for copping an arrogant attitude, taking my money, giving me change, and pushing a paper cup across the counter for me to fill myself.