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Are vB forums even able to have a "Like" button? Only thing I know of that is similar to that is the Reputation System(which all staff are strongly against).
It's all gonna be a popularity contest. "X is my friend, so I'll just like his posts." Or a way to justify arguments in say the real world. "Dude, my post has more likes than you, therefore I am right." Maybe not as much like that, but I think it will influence it in some sort of way.
You know, I can even see it creating more spam. In the middle of a thread people will start asking why their post didn't get as much likes as the other guys post, while they said the same thing.
I don't see anything good coming out of a like-button.
I vote DOWN. If Youtube's taught me anything, this idea could just end up as a popularity contest with people just commenting on the results. ^^;