No game review can possibly be "unbiased." I personally believe complete objectivity is impossible, because logic is always based upon our own personal core assumptions.
However, I think a really bad game review ONLY offers opinions rather than saying WHY. For example "This game sux!" or "This is AWESOME!" Even if the review goes on for paragraphs and pages, if it ONLY offers the reviewer's opinions and doesn't tell me ABOUT the game, I think it's a waste of a review.
A good review obviously has opinions. These are really helpful when combined with the specifics that led the reviewer to that conclusion. If I reviewed a really crappy game, like the "Block Land" game (it's on Youtube), I'd say something like this:
"The game attempts to make simple custom graphics, which is a plus. Unfortunately, the graphics themselves seem buggy (face portraits are chopped off, tilesets seem improperly fit). The text in the game had a large number of typos and grammatical errors, which made it hard for me to follow. Combat in the game was boring because it was extremely unbalanced --- most fights were 'you have to TRY to die' easy, rendering it boring.
The maps, overall, reminded me of the classic Adventure game for the Atari 2600 --- very blocky. While this was justified in-universe, which was a plus, it meant the exploring felt odd.
Overall, it had some interesting concepts, but needs a LOT of polish, re-balancing and bug fixing to reach its potential. Right now, I wouldn't play the game again, and I would be very reluctant to play future games by this developer, because the game didn't seem to be completed, yet was listed as completed."
I consider THAT to be an example of a good review of a very bad game. Sure the reviewer has his opinion on the subject, but he gives reasons WHY he feels the way he does.
Bad reviews are SOLELY opinion with NO information about the game itself. People read reviews to find out information about the game, so a bad review is just opinion.