I'd point out that most people who play games rarely finish them, or chase down every achievement. Like it or not, not every game manages to keep the people playing interested enough in order to finish it. I think most gamers will talk about having huge piles of unfinished games.
Especially with their schedule and professional requirements, reviewers can't be expected to play every through to completion in paticular. However they can be expected to put enough time in to form a fair opinion. Truthfully I think a few hours is frequently enough time to get a fair impression from a game. Especially seeing like writing novels, if the people are doing their job well there should be something of a hook to get people interested and keep them there even through slower points of development.
If a game reviewer can't get a fair impression of a game in a dozen hours or so that they are probably going to be putting in, then I think that this is in of itself a problem with the games.
What's more I think genere has a lot to do with it. Having a critic like Yahtzee review an RTS or RPG when he hates those generes and everything about them, and considers a lot of things that people who like those generes to be consider virtues to be an anethma, is counter productive... he can't tell you if something is a good RTS or RPG based on his hatred of the generes and their conventions. The same could be said of reviewers who hate shooters or fighting games or whatever and find then equally banal. It is also noteworthy that a long, epic, slow-burn RPG is going to take longer to review even if not finished than a shooter that has a 4 hour single player campaign and relies on "multiplayer" to justify it's existance. Of course in such cases it's impossible to fairly review a game like that because how a "controlled" enviroment of multiplayer only being experimented with by reviewers and the like before release, there is no way to tell how it will work with the community at large... the "human factor" able to destroy many concepts that work well on paper or in testing.
Of course at the same time I am a bit wary of game developers having any more influance over reviewers than they do now. As it is I've already heard of them requesting specific types of reviewers for their products and so on. Not to mention the whole Gerstmann thing and what that said about the way advertising money is influancing the impartial nature of reviews. On some levels I have an instinctive reaction to want to scream "no" to any request made by developers here no matter how reasonable seeming.
I'll also be honest in saying that I have also been becoming a lot more jaded as far as games go. Simply put I think the gaming industry has gotten more and more deceptive about marketing games, where once I could understanding "holding the cards close to their chest" to some extent, more and more frequently it seems that we're not seeing much accurate gameplay footage until games are released, and crucial features are being removed without that becoming general knowlege. To put things into perspective despite all the MMORPG burns of the last few years, the general populance has very little knowlege of what "DC Universe Online" is actually like, all we really know is that they can make a decent looking (if stereotypical) 5-6 minute cartoon. The reviewers have already been somewhat compromised, and honestly as game consumers they are increasingly becoming the only thing we have left for game information before actually handing over money to try it ourselves.