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To review a piece of work (movie, game, art piece, etc) is not just to give your own opinion, informed or otherwise, though that in itself is a general basis of a review. In fact, a good reviewer will always have their own experience put aside as an example not the basis of the work itself. A good review says what is good and what is bad and everything in between, giving the right amount of information (plus the author's own experience as an example rather than the basis of the review) for the audience to determine their own personal opinion on the product.
This relies mostly on the one reading/viewing the review as having enough knowledge about their own likes and dislikes, as well as being capable of formulating an opinion based on the information given. It is the fault of the review for not assuming these factors and instead blatantly instructing their audience on how to think and act (however few people are capable of being otherwise, and instead actively looking for such reviews).

Personally I believe a review must give me enough information as to wether or not I wish to buy/see the item in question, wether it is worth my time and money, and to not insult my intelligence or already formulated opinions of the product.
Many movie reviewers, for example, in their attempt to seem 'hip' to their postulated audience, will instead insult or offend anyone who would have gone to see it without the review's efforts (see: 'Astroboy' Reviewer).
A good game review, in comparison, will emphasize wether or not the gameplay itself is of interest to those viewing the review, based on game-genre, interactive medium, story and atmosphere. These things are difficult enough to create without their description being only subpar in comparison.

In conclusion:
Don't review just because you hated/loved the thing. Give us, as your audience, an idea if we will love/hate it ourselves.