I also stress the importance of answering "Why?" I love hearing and letting them express their individual opinions on works we read: they come from so many different backgrounds and experiences--even at their young ages. However, I'm trying to also teach my students the importance of specifics and support for their opinions. Everyone has an opinion, but how long do you listen to someone who can't explain why they have this opinion? Posing that question to my students (no matter the age), really makes them think about the importance of support. This translates into their writings also.
The previous answers were quite lucid. I think the reader response model can take on several forms that allow students to demonstrate their understanding of both content and themselves. I like the idea of having students create a "character backpack," where they have to internalize a character in the novel and pack a backpack of items that would represent or do represent that character. It allows students to connect with the characters in novels in a very direct and responsive manner. Another particular task would be to create a CD or playlist that reflects a character, as it helps students respond to literature through music. I also like the idea of having students select a scene and act it out in their own interpretation of how the scene would look and how the characters would act. In each of these, individual students are able to respond to literature and characterizations in a personal and intellectual manner.
Middle school children also appreciate a kinaesthetic approach to reader-response approach. Voting on particular ideas or character actions can work well, as can in-role questioning of characters (sometimes better if this is very teacher led!).
Middle school children can make very astute observations and are generally able to engage in discussion surrounding choices and decision making by characters and how this may fit with their own moral codes and ideals. I have had really involved class discussions around novels such as 'Abomination' by Robert Swindells and student reactions to the characters of Martha and Scott and their backgrounds.
Great question. Isn't almost all discussions with children reader-response? Whenever we talk about what they feel about the story rather than ask what the text says or what the author intends, we are doing some reader response approach. In the light of this, I would simply ask people for their opinions and ask whether the other students agree or disagree. If the children are lively, then it should be a good discussion. You might also have them draw a pictures of what they read or better yet a cartoon strip. I am sure that you will get wildly different pictures and cartoons!