I know from personal experience that they DO want to hear feedback. However, as with any restaurant, there is an appropriate channel for providing that feedback.
First, you need to express your dissatisfaction to the server. That person is your personal liaison to the restaurant. If you don't get satisfaction from that, then you need to discuss the situation with the manager of the restaurant, at the time the problem is in evidence. That should address the issue, especially at WDW. They have some of the most courteous and helpful restaurant managers and executive chefs I have ever encountered.
After you leave the restaurant, the opportunities to provide feedback are less vital. This is because of a long history of abuse of Disney's generous guest recovery program by guests. The PO Box mentioned earlier for Guest Communications is the channel at that part of the process but be prepared to have to have your issue addressed in a completely different manner, with courtesy, yes, but without any real means of validating the issue, many folks find that complaining after-the-fact doesn't yield the kind of results they were aiming for.