I'm glad I haven't read this thread, but I'll just throw in a bit of food for thought (HAR HAR). The way I see it, is that tip bottom line % is more based on the type of establishment you're visiting. If you're at Denny's or iHop, the average tip % at a place like that is probably 11-12%, obviously depending on many factors. A midscale place, like a Chili's or Applebees or a pub type place, I'd say servers probably expect and perform at the 15% range. The places I've worked more recently in my career (after almost 8 years of experience in my field), where you usually eat no less than 3 courses and your server constantly makes sure your wine glass is at its sweet spot of fullness, 18% should be the baseline.
I actually go the opposite way on this.
At a classy restaurant, the higher price of the food (and alcohol!) are inherently causing that 15% to be an appopriate amount for that experienced server. Yeah, 3 courses takes more work, but it's also adding to the bill that the tip is based on.
Whereas at Denny's, even if I just get an $8 grand slam, the server still has to make ~4 trips to the table, so tipping $1.20 is pretty shitty. Also, bumping up to 20% on an $8 bill is trivial. Also, a Denny's server is probably poor, so they get some guilt-tip from me.
Actually, I'd think the quality difference of a Denny's vs. a five-star would inherently affect the quality of the server and the increase in tip. Foul-mouth Flo in her hairnet probably won't be worth more than 15% whereas Jacques with his silk tie is more likely to provide a 20% service.