I've long wondered about this question. Say that you normally tip 15% for normal good service in a full-service restaurant. How much should you tip in a restaurant where pretty much everyone eats from a buffet? The waiter takes your drink order, gets you the drink, and gets you the check; the waiter or the busboys clean up after you; the waiter or a busboy takes care of small condiments requests (e.g., to get a bottle of low-sodium soy sauce or stuff like that). Should it still be 15%? 10%, because the waiters do much less than full-service waiters do, and end up serving many more tables? 5%?
More to the point, what's the social norm? There's nothing magical about a 15% tip, or an 18% or 20% tip if some people prefer it — the important thing is that it's a social norm, and becomes part of the deal: You go to a normal restaurant, and you're essentially (morally, if not legally, though there's been one odd legal incident that I know of related to this) promising to pay the cost of the meal plus 15%, unless the waiter's poor service gives you good reason to undertip. What are you promising when you go to a buffet?
I'd love to hear people's views, especially on the descriptive what's-the-social-norm question.
UPDATE: If you're concerned about the waiter's income, keep in mind that because the waiter has to spend less time per table, he will probably be serving more tables. A waiter who is tipped 15% per order at a buffet service restaurant will thus end up making more for the same amount of total work (work per table x number of tables) than a waiter who is tipped 15% per order at a full service restaurant. (The busboys still work pretty much as hard per table, though, and I assume that the tip is split with the busboys, so you can't just proportionately reduce the tip by the reduction in the waiter's amount of work.)
On the other hand, I don't know to what extent this is taken into account in the base salary that the waiter is paid by the restaurant; that, I take it, turns on what the restaurant and the waiters expect the tips to be. And I also have no personal experience confirming my supposition that waiters at buffet restaurants serve more tables than at full-service restaurants, though I think my speculation on this is well-founded. So none of this proves that the rate should be below 15%, but it does suggest that the "buffet restaurant waiters need to live just like full-service restaurant waiters, so they should get the same percentage tip" argument is likely mistaken or at least incomplete.