For a site like IMDB in which users write reviews, is it preferable to allow users "Like" a review( e.g. goodreads.com) or answer "y/n" to the question "Did you find this review helpful?"?
I think "Did you find this review is more helpful" is more precise, and attempts to solve the disconnect between negative sentiment and a "like" action. In other words, a user may really dislike a product or dislike a reviewer's writing style, quality, etc. but at the end of the day, the review was helpful in their decision whether or not to purchase the product. So the user can comfortably say "this helped me" even if they can't say "I liked this"
I don't suppose you've ever read Jared Spool's The Magic Behind Amazon's 2.7 Billion Dollar Question?
In times like these I would recommend looking to those that have done the research before you, such as Amazon. According to Mr. Spool, the question "Was this review helpful to you?" generated Amazon $2.7 billion in revenue. While they didn't actively consider a like button, the article does discuss a yes or no feature and plays on the art of good design. My recommendation would be to put a "y/n" feature to the question "Did you find this review helpful?" as this article describes.
I can like something without it being helpful.
"Did you find this review helpful?" is more direct and will give you a relevant answer. Whereas "Did you like this review?" can give you the wrong information. I sometimes "Like" things, not because I actually found them helpful, but because I do know that it's a popular topic that many others are interested in, so why not vote for it? Also, I may like a review that wasn't helpful. It may not have been what I had expected, and thus didn't answer my questions, but it may have been good enough to distract me from what I intended to read it for.
If you want to know if something is helpful, then ask a direct question: "Was this review helpful?"
If you want to know if they liked the review, say "Did you like this review?"
Ideally, prompt the user to think in terms of whether the review is helpful or useful in making a decision.
There are a lot of reviewers who make silly jokes, angry rants, or spout on political views. A significant number of users might enjoy reading those reviews, even though they may be irrelevant to the subject and get in the way of a more serious user's research.
I'd avoid terms like Like/Dislike or Agree/Disagree so that reviews are less likely to be promoted or buried for the wrong reasons.
As Daniel Zahra said, simple up/down arrows work well, but I would consider adding hover texts. Something like "this comment is helpful"/"this comment doesn't contribute to the conversation" (but for reviews).
I have found it helps for me if I am about to "misuse" voting systems by upvoting things I just find funny or personally agree with.