We wrote once regarding water in our cabin - two decks up. We wrote not because it happened but because of the actions of the pursers' desk. We did receive a 10% discount on our next cruise.
We have also written to praise crew that provided excellent service when provided - and no comment card was made available.
A Bad Day At Sea [with power] Always Beats A Good Day At Work
Carnival: Glory 2004, Destiny 2008, Splendor 2009, Freedom 2011, Valor 2012, Dream 2013
Celebrity: Summit 2011
Princess: Ruby 2010, 2014, Caribbean 2013, Coral 2014