I can't fathom what would make people think that a "like" button would be a good idea. It works on sites where people don't know each other (such as YouTube) and it works on sites where people share things (such as sites where people upload fanfiction, etc).
But on average internet forums where people who know each other discuss opinions, the only thing that happens is that people vote up comments made by people they like and vote down comments made by people they don't like, without any regard to the actual quality or usefulness of the comment. I can just imagine what will happen when there's a flame war (since I've seen it on other forums); two factions will emerge and one faction will vote up every comment made by the person they support (without even reading it) and the other faction will vote up every comment made by the other person.
Having a like button would just turn everything into a popularity contest and certainly wouldn't reward intelligent comments that make people think or force them to challenge their preconceptions.
