It's either a request for a dislike button or a "Jesus, you're a stupid [****][****][****][****]er!" button.
It's a double edged sword; with the post ratings mod I've seen the 'Like' function increased but in some case I feel it is at the expense of people posting replies.
Might be a bad idea to allow negative rep/dislikes. Seems too easy to abuse, especially in the vBulletin/Internet Brands topics (by both sides).
I'm generally opposed but my thought process was that it might save the rash of "You're a [****][****][****][****]tard!" posts that usually occur in those threads. Every discussion ends up derailed. Even less a fan of that than I am of dislikes.
I try to have fun with those issues by using a disallowed words list where disallowed words get replaced by words of my choosing. So, for "[****][****][****][****]tard" I would probably set it to be automatically replaced by "sweetheart". "You're a sweetheart!" teaches the members to control the language and causes a few laughs at the same time. I think that a discussion forum should breed discussion and something like a Dislike button would probably dilute this.
I am highly against thanks/like/dislike/hate buttons. If you want to dislike what someone says post and give the reasoning for doing so. More content is generated without these types of mods. More content = happy forum. I am all for those Username vs Username posts on a forum, I believe those help the two people say what they want and when its over its over. Let them go at it and get it out of their system
I'm not that fussed, but if having a choice would prefer both. But only if there was an option like MyBB has to limit "based on usergroups" you set a limit, how many Likes/Dislikes can be given in a 24 hour period. At least that stops people from spamming LIKES and would also make people think twice about wasting them on posts given - if they can only hand out so many in 24 hours.
I have no issue with posting "Jesus, you're a stupid [****][****][****][****]er!" if the situation calls for it. It's just that some might take that as baiting rather than constructive criticism to learn something about the discussion topic before posting.