Anyone who has read one of my reviews knows I'm something of a cynic and a pessimist. I try to think the best about every VN I go into, but my first impulse is to see what is wrong, rather than what is right.
Whether it is optimism and rose-colored glasses or pessimism and cynicism, and excess of either is often a negative influence on the quality of a review. Generally speaking, I usually make an effort to find something I like about a VN's concept before going in, then I start the VN trying to enjoy it as an outgrowth of that. By the end, this usually results in me having experienced both the negative and positive aspects of the VN... the problem is, when reviewing, it is all too easy to forget what is good about the VN.
As a result, when I'm writing up a review, the first thing I do is write up a list of the good points I found, ignoring the mitigating negative factors. I then build the review around these and include the negative points in with the rest... but you can probably tell that being positive just doesn't come naturally to me, since I tend to be pretty harsh.
However, by using this system, I've found dozens of VN gems over the years that I probably would have discarded for perceived negative qualities if I didn't use this process. Indeed, early on in my reading of untranslated VNs, I dropped numerous ones simply because they had a negative aspect that I got obsessed with. I would later go back and replay them, only to find that the negative aspect wasn't as big a deal as I thought at the time, since I made the effort to go back with a differing perspective.
A poor quality in a reviewer is the tendency to ignore the negatives about something you like. Another one is to rate things entirely based on aspects you only have a vague grasp or focus on (in my case, due to my eye problems, I'm not the best judge of artwork, and my musical sense is entirely based on how it enhances the atmosphere, rather than raw quality comprehension). I'm a story reviewer. I review almost exclusively based on the story, characters, and presentation. As such, art and sound rarely have a place in my reviews, since I don't think I'm qualified to evaluate them except in the most general of terms.
I can tell when a VA did an exceptional job, because it stands out enough for me to notice. I will even mention this in the review, since it takes a lot for a performance to stand out to me. However, I never pretend to know the ins and outs of specific aspects of VA or musical quality. I simply don't have the right kind of ear for that kind of thing, not being musically inclined.
One thing I've noticed in some reviewers who prefer niche genres (such as myself) is to display a tendency I refer to as PGRD (or Popular Game Reactionary Disorder). It is a fictional mental disease that many of us who have a distinct preference for a niche genre display that causes us to have a knee-jerk negative reaction to popular works, simply because they are mainstream. This is a problem that is particularly common in Western otakus of around my age, who became fanboys during a time when watching anime, playing Japanese video games, and reading manga had a rather strong stigma that left us feeling isolated and defensive. However, it is also present in people who prefer niche genres (I get the double whammy, being both). That sense of isolation leads to a tendency to over-praise our favorite materials and bash anything that we see as being too popular.
In reverse, there are those who automatically dismiss anything that isn't mainstream. Both types are reactionary in nature and have little to do with the quality of the materials in question. Being a long-time sci-fi addict, I can't understand why anyone would enjoy Avatar (the movie). However, if I make the mistake of saying that in front of a fanboy of the movie, I will inevitably get a vociferous lecture on how misunderstood the movie is by science fiction fans...
There are many such examples of such behavior I have experienced over the years, both in myself and in others. As such, a reviewer has to be willing to examine his own motives for liking or hating something. Are you being cynical for the sake of being cynical? Are you over-praising something to the point of overlooking the obvious problems with it? Are you making excuses while thinking you are making a reasoned argument? On the other side, are you ignoring the voice of reason to give you an excuse to dislike something?
In the end, bias is unavoidable... but it is a reviewer's duty to do their best to cast aside as much of it as possible, because people use our reviews as reference points when they pick what they want to play/read/watch.