I have posted a few reviews on trip advisor, good and bad. I always try to be balanced when posting reviews and mention positive things as well as the negative, this makes the review more believable than if is is all bad or good. If you really cannot find anything good at all to say, then as buenchico advises, back up your comments with the good sound reasons why you have given the bad review. I once stayed in a very poor hotel and after posting my review on tripadvisor, was contacted by a viewer to say that she had cancelled her stay with the hotel on the strength of my comments! So I felt I had done my good deed for the day!