Tripadvisor is fast becoming a trusted source for research as well as on-line booking. So, why wouldn't you hand out a card to every guest with the URL leading them to review your hotel? Maybe even give them a card at check-in so they can post during their stay. Seems like a better use of time than a long comment card to fill-out. Just a thought.
Oh, the more reviews you receive, the higher you rank on search engines.