The always-interesting--really!--Joel Spolsky lists his "Seven Steps to Remarkable Customer Service".
When customers have a problem and you fix it, they’re actually going to be even more satisfied than if they never had a problem in the first place.
It has to do with expectations. Most people’s experience with tech support and customer service comes from airlines, telephone companies, cable companies, and ISPs, all of whom provide generally awful customer service. It’s so bad you don’t even bother calling any more, do you? So when someone calls Fog Creek, and immediately gets through to a human, with no voice mail or phone menus, and that person turns out to be nice and friendly and actually solves their problem, they’re apt to think even more highly of us than someone who never had the opportunity to interact with us and just assumes that we’re average.