My best friend has been back from her honeymoon for only a few weeks, and she already has another trip on her brain. The difference between her honeymoon and the next vacation? She and her new husband will be treated to a complimentary hotel stay on their next trip! Because of the simple act of writing a complaint letter, they were offered a free five-day / five-night stay at one of the resort's hotels anywhere in the world.
While their Tahitian honeymoon was nothing short of incredible, they had issues throughout their stay with icy cold water in the shower and spa. When they asked about the problem, hotel staff ensured it would be fixed, but they never felt anything resembling warm water.
Upon their return, my friend wrote a letter to the hotel stating her disappointment with the issue and how it was handled. Her recommendation is to give a letter of complaint to your travel agent (if you're using one) to add a cover letter before it's sent off to the hotel for an extra punch. Her situation shows that if you have a real issue that detracted from your experience, it can pay to complain.