I do not see any benefit, and plenty of possible problems with, a "dislike" or "unhelpful" button in the context of a conversational forum.

Given that a separate Q&A section isn't in the cards, I don't think there's any reason to consider that. FWIW, I'd prefer to see a "like"/"thanks"/"helpful" option be non-anonymous (even if you have to click through to see who "liked" it).

As for a difference in who's posted in favor, and the vote counts, look at it this way: one of the benefits for a "like" button is to give a way for people who are less inclined to post a way of saying they appreciated a post. Arguably, it's the same with voting: it's much easier to click "yes" and "submit" on the vote than it is to actually write out an argument in favor.