Great care should be taken when choosing which symbol to use as you don't want to encourage votes based on emotions of content, rather the quality of that content.
As a morbid example:
Facebook status wasting internet:
Oh no my dog is dead. What a shame. May he RIP.
What would a 'like' of this suggest? Do you like the fact their dog is dead? Probably not. Do you want their late dog to RIP? Probably. How do you display that emotion?
Consider another example - a youtube video showing information about something bad, to use current affairs, the Norway Gunman's trail. This has like and dislike options. You may LIKE the content of the video (Journalistic quality, production) but naturally you dislike the subject of the video. Does that warrant a like or a dislike?