Why are you leaving? What sort of job are you going to? Is there anything we could have done that would have persuaded you to stay? (assuming they are sorry to see you go). Are you clear about your pension / deferred benfits entitlements?
You could say as much or as little as you wish (or indeed, nothing) - much depends on the relationship you have had with your employer/manager.
I was given feedback on what they thought of me, and asked for some of what I thought of them, as it was an awful place to work it wasn't very pleasant, but I'm sure you won't have that experience