I'm sorry, but, there is a difference between pre-emptively establishing an opinion of a game for the purposes of making a wise purchase.. and shoving that opinion down people's throats, or misrepresenting its weight.
If you've decided you don't like a game prior to buying it, based on available information in the world at large, then.. yeah, goodo. We all do that sometimes. If someone asks you if you intend to buy it, well of course you're justified in saying no, and maybe explaining a bit, if you fancy.
But a review without touching a game? That's screwy. Now, sure, not playing the whole game through is understandable - a game might get wonderful in the later part, but it's a failing on the creator's part if they made the first half bad and put you off. If a decent stab was made, then all well and good.
Going onto a game's fansite and bashing the game without playing it? That's .. well yeah that's hater territory, and stupid. .. Actually so's going onto a fansite and bashing the game it's for having played it is iffy, too, come to think, what with it being a fansite and all.. but whatever.
There's a .. well, not a fine line, really, but a line anyway, and while having an opinion on a game without playing it or having only briefly played it is all well and good, the manner in which you deploy that opinion is not automatically justified. Now, what you review's you're own lookout and people reading that is their's, but all the same, what is unprofessional (note, not accusing anyone of anything, just rounding up this silly little post) is failing to warn at some point - preferably the start - that the opinion in the review is formed from an incomplete view of the product.
Honestly I think that's just good writing practice - not misleading people, I mean. Doing the game in question justice has nothing to do with it, and I wish all reviewers would point out how much of the game they've experienced.
... I really shouldn't try to post after midnight x.x