Aside from giving the user feedback, I find it helpful when forms help fill themselves out. For example, using ajax to suggest the city name when the zip code

Aside from giving the user feedback, I find it helpful when forms help fill themselves out. For example, using ajax to suggest the city name when the zip code is entered, or only showing followup questions when a certain response to a previous question is given. I used this in a survey application I made...if users rated 3 or below on a 1-5 scale, a "please explain" box appeared.