Well, to be fair, it wasn't so much that you said table servers should be tipped higher than buffets - it was the clear implication that buffet servers should be tipped less than 18% that set people off. Otherwise, as another poster pointed out, why ask the question at all. Since you normally tip 20-25% at table service restaurants, you will be saving money at the buffets. We've made over 50 trips to Disney and it's been our experience that buffet servers work just as hard as table servers, although their jobs may encompass different responsibilities. In fact, in many restaurants, table servers do not even bring the food to your table - it's done by food runners. Your experiences are different, which is fine, but I think the strong responses were due to the feeling that 18% really isn't so much of a tip that people should be looking to lower it.
I imagine the majority of people tip less than 18% at a buffet.