At almost every WDW restaurant you can wear what you'd wear to the parks, including t-shirts and shorts. Victoria & Albert's is the only place that really requires dressier clothes - jackets for men, etc. A couple of the nicer restaurants (Calif. Grill, Bistro de Paris, maybe others) don't allow tank tops. You might see quite a few people in some of the nicer restaurants in polo shirts and khakis, but it's by no means required and you'll probably see a wide range of attire wherever you go.