I really don't think people have to be "dressed up" to show proper respect for the environment they are in at a WDW restaurant. In fact, I would much rather see well mannered people in shorts and polos than the group of loud, obnoxious, drunken conventioneers (who were dressed in slacks, dress shirts, and sport coats) we had to deal with the last time we were there. Granted, they were probably the most appropriately dressed group in the entire restaurant, but they acted the least appropriately, IMHO.
However, I do think the basic requirements of "resort casual" should be met--no tank tops, no swimsuits, no flipflops, no cutoffs. All this is JMHO, of course!