1. Captain Coupon

It's always a bad sign when someone orders a fountain drink and asks, "Are there free refills?" It's usually a guy out on a date, and the girl always looks embarrassed. He takes great pride in trying to manipulate the items on the menu for any loopholes and challenges me on why things are priced the way they are (thanks, Matlock, your date is really impressed). He'll proceed to eat 4/5ths of his plate and ask to have it refunded from the check because he didn't enjoy it and has lost his appetite. Then he'll pull out a coupon for a free dessert or x amount off the final bill, tipping exactly 15% on the revised amount. Out of spite I make it my mission to fuck his girlfriend.
2. Old People

It's not 1952 anymore, grandma. An average tip isn't 10% anymore. Also, please stop using coins. I really don't care about your cat or great-nephew. What I'm concerned about is that you're taking up one of my biggest, money-making booths during rush hour because you wanted to sit in the quiet corner, you both ordered waters, you're sharing one $8 salad, and it's going to take you 45 minutes to finish it. I'm young and still have plenty of stupid things to accomplish with my life. And no, I don't want to tell you about any of it. Why? Because I know you won't tip more than 10% despite whatever rapport we build. Worse, I learned early on that you grow attached to me and request to be seated in my section, only to repeat the horror of your life's minutia and shitty tips ad nauseum.
3. Verbal Tippers

As a seasoned, veteran server, nothing burns a hole in the pit of my stomach like having someone I just waited on come up to me, offer a handshake, and thank me for my excellent service. When I first began serving tables, I'd receive these pleasant gestures and gingerly make my way to pick up the checkbook, all sorts of shimmering percentages dancing in my head. Then the shot to the gut...12%?? And over and over again. Even 15%. What teacher commends you for excellent work then hands you a C? It's all about setting the expectation level; at least bad tippers who avoid eye contact when they leave have the decency not to get our hopes up.
As I see it, verbal tippers are cowards. They hide behind their words. They're fully aware of their meager pittance and try to cover it up. That, or they're just plain retarded and think we work to feel pleasant about ourselves. Sorry to break it to you, but we work to ...earn money? What a concept!
4. Collection Plate Tippers

This is another one that has everything to do with expectations. (Just as an aside, I should explain that Sundays are called Sunday, Bloody Sunday for good reason. What can go wrong, always goes wrong; it's Murphy's Law on crack.) So you bounce in to work, miss all the NFL action for the day, and welcome in all the happy little Protestant families and church groups that decide a lunch is in order after morning ceremonies. They're all dressed very well, polite as ever, and in genuinely good spirits. Everything at the table miraculously runs smoothly, congenial conversation takes place, and the smallest thing you have to worry about is how to respond to their awkward, persistent smiling. Somehow, this family's disarmed you into believing this won't be like every other Sunday. Then you pick up the check...$12 on a $120 bill? Jesus Christ! (A propos.) It's not before long you've figured out this pattern, and the conclusion to draw is simple: where's that extra $5-10 missing from the tip? Ahh yes, it got lost in the collection plate being passed around that morning. You see, they've done their duty to God for the week, so they've afforded themselves the luxury of being douchebags for the next seven days.
5. High-Maintenance Types

There's two classes of high maintenance people: those who are cognizant of their own demands on other people, and those too caught up in their own little world. The former has the potential to be polite and tactful, using such tag statements as, "I'm sorry to be such a bother," or "If it's not too much to ask." They also tend to tip in kind, meaning I never have a problem working hard for them. The self-absorbed variety are a whole other species. These are the people who make you feel like you've adopted your job title -- you're there to "serve." You bust your ass for them and they don't reward you for the extra effort. These people are also retards, the types who'll frequent Pizza Hut and ask what else in on the menu.
6. Low-Maintenance Types

A contrast to the above, nothing can give a server a bigger headache than someone who is too laid-back. You'll usually know these types because they'll ask inane questions like, "What do you guys have to drink?" with the menu closed in front of them. I'll answer that one with, "Liquids...mostly," laugh as I walk off, then go cry in a corner. They also take about 20 minutes to order. Do you want fries or mashed potatoes with that? "Hmm....uhhh ....... ....... uhh, whatever..." What I will give them, though, is that they typically tip pretty well and remain relaxed despite all the shit that can go wrong with their meal.
7. Indecisive Schmucks

The same problem as the low-maintenance type, only far more frustrating. The rest of the tables sits there empathizing with your buckling knees as Felicia Fickle seeks a 4th opinion. Before you know, you're running back and forth to the kitchen several times to amend their order. A server's best friend is looking around and saying, "I'll be right back." Learn to use it often.
8. Mr. Hook-It-Up

"Hey bro, can you hook me up with anything? I'll make it worth your while." Big surprise here...you expect this cheap ass to split the savings with you? Worse, if you don't hook him up, he'll mark it as a score against you.
9. Campers

I can totally understand vegging out after a big meal or enjoying the company of friends you haven't caught up with in years, but please be aware of how it affects the server's income. Our salaries are based almost strictly on tips earned, and our tables work as a form of real estate. The more people we can get in and out of there, the more product we can move, the more we stand to make for the night. In the two hours you just spent waxing polemics, we could have turned the table over twice and pocketed an extra $20 for the night. Some people are beautiful and leave extra money on the table for us. This "Fuck You!" goes out to those too self-absorbed to recognize their impact on others trying to earn a living at their expense.
10. Mrs. Q&A

How the hell did a five year old take the form of a grown adult? Combined with the Indecisive Schmuck and High-Maintenance Type, you're in for a real world of hurt. "Can you tell me about this chicken dish? ... Can I get spinach instead? ... Can you make it without salt? ... Is it really good? ... Ok, thank you. ... Now can you tell me about this pasta?" Meanwhile I'm looking at my other tables as they're beckoning for me, and I'm stuck in the wrath of this unholy cross-examination. I make it a point to visit these tables as infrequently as possible.