The Rule of 15

There is one societal exercise that says more about us as a culture than anything else we do: tipping. Initially it was tied to how well a particular person did with relation to their job. If you're at a restaurant and the waiter came, gave you the menus, left, came back an hour later, got your orders, left for an hour, brought them back (wrong), brought the check with it, and was surly in the process, you'd give them nothing. It's an incentive, and its genesis was in the American belief that you control your destiny.
I've been to several different countries, and the US is the only one that tips. However, it seems like in recent years the tip has become more of an obligation and less of an incentive tied to the service level provided. For example, in most chain restaurants if you have a party of more than 6, you get hit with an 18% "service charge." Why is this? I understand that waiters need to earn a living and the restaurant certainly isn't helping them. They earn $2.35/hour and then subsist on tips. Nevertheless, it no longer provides an incentive for the waiter. They can't make more than that (well, they can, but who tips on top of a forced tip?), and they can't make less. It eliminates the whole point of tipping.
Another awkward situation is what some restaurants put their customers. The most notable example that I saw was at Fuddrucker's (although it seems they have eliminated this practice), although it's prevalent at all-you-can-eat buffets as well. It's the "waiter" who doesn't do any waiting. You pay for your food up front, you get your food and beverages, you do everything yourself...and they have someone come by and ask how you are and if they can get you anything. What do you tip these poor souls? I usually only carry plastic, so if I've already paid up front (and there's no spot for a tip on that reciept anyway), what do I do? I'll tell you what I do. I eat, then I get out of there as soon as I can. There really should be a law against that.
What do I tip when I go to a restaurant? I generally have a three-tier system:
Tier 1: 10%. You have to be pretty pathetic to only get 10%. This is if you are exceptionally slow, or if I go for 30 minutes without anything to drink. Seriously, I know I drink a lot of water at restaurants, but I should never have to wait that long for water. All you have to do is swing by - heck, just give me a pitcher. Don't leave me hanging. There is a chance that you could get 0, but since I've never given a waiter a $0 tip, it's not a separate tier.
Tier 2: 15%. This is for average service. If you do everything pretty well but aren't exceptional at all, you'll get this. It reflects some work on the part of the waiter, but just enough to be considered nondescript. These waiters are the vast majority of the crowd, and you wouldn't remember them or recognize them if you passed them on the street.
Tier 3: 20-25%. This is for above average-excellent service. I typically also give an average waiter 20% if I'm on the company's dime during my meal. If you've gone above and beyond by being quick, personable, friendly, or doing anything more than that, you'll hit this range. There are some outliers - I had one waiter who was so good in Orlando and the bill was so small (it was Macaroni Grill, and I was on business) that he literally got a 100% tip. He was the best waiter that trip, and it included some expensive haute cuisine that should have given us better service. Nevertheless, in general I will reward someone for doing a great job, because that's how it was designed, and because I want to teach the world to sing in perfect harmony. Something something something something, make waiters better in general.