I was listening to a podcast the other day where some writers and editors for prominent game publications talked about how many gamers put too much stock in game previews. The argument was that for the most part previews have to be optimistic given that they are looks at incomplete products and nothing is written in stone.
For the most part this is a good position to take; previews can't look into the future and tell you what the final game will be like. Features my be added or dropped, bugs may crop up at the last moment or problems with framerates may be solved. A game that gets a glowing preview may end up being a total turd while one that leaves middling impression could turn out to be the second coming of Donkey Kong. It's not an exact science, anyone who has done extensive beta testing can tell you that. Developers get the benefit of the doubt because they're people too and a product isn't done until it is sitting on store shelves.
Still there is a problem. Previews come too early, but reviews come too late. By the time you find out that a game like Katamari Damacy is totally amazing you are already relegated to the retail store crap shoot. GameStop won't have a copy for you because you didn't pre-order and the big box stores probably didn't order any because it's too weird. In a business where profit margins per unit are razor thin pre-orders are going to be the law of the land. Previews need to reflect that.
For a game that is reasonably popular there's no need to put money down; they'll have enough copies even if they swear to be selling you "the last one." But for games like Okami and Zack and Wiki chances are that your store will only get enough to fill pre-orders. These are the games where previews need to function as pre-release reviews, where writers with review copies need to give estimated scores before the final review is published. People need to know about innovative and unique games before they hit the shelves so they can put their five dollars down. A preview can't always be a preview, sometimes it has to be a pre-review judgement.