It's easy to thank customers for their positive feedback. When someone gives you a glowing review or tweets about how awesome you are, it is natural to thank them because they gave you a good feeling.
Thanking customers for negative feedback is even more important, though it feels less natural. But think about it. Learning from customers about what they don't like or what doesn't work is even more important. You take that information, improve, and grow.
Here's are some under appreciated people that need to be thanked:
When you take the feedback and make it actionable, you are going to get better. Many times the people who give you negative feedback turn into your biggest champions when the feel they are being heard.