I won't bother listing games and sources as we all have some bias against certain sites because we think they are biased against our certain console.

The real question is, how many hours do they put into playing the game to review it and score it properly?

Sandbox games are not a game that you can "complete" so those are expection and one doesn't need to complete it to give a "fair" review.  However, how many hours do you think is proper?

Meanwhile, games that require 15+ hours to "fully" complete, I think, are necessary for a play through in order to give a "fair" review.

I've read my fair share of numerous reviews. In most of them, they pretty much say the "caption obvious" things instead of the actual experience while playing. Did they laugh? Did they cry? How many times did they have an adrenaline rush dodging those hord of zombies? Did they feel attached to certain character after the game? They don't need to be specific in details. This gives me an impression that they spend about 2 hours reviewing the game instead of a play through. 

I think they *should* complete games (as you said, it is hard to do so with sandbox games, but other ones), but whether they do or not is another matter

The definition of "complete" is pretty hard to define. With an online oriented game for instance, especially MMO's, I don't think they could fairly rate the game unless they have played it for several months.

I can define "completing a game" pretty easily actually. They have completed the game when they have accomplished the major objectives set by the game AND no longer feel an urge to play it on a semi-regular basis. Thats when someone has pretty much completed a game for most purposes. Although in some cases...like online only games...that could take a while so it might be unreasonable to review it. The reason I would use this standard is because that would be the point that a normal person would no longer be playing the game so they would then have the full perspective of the game that they are going to get.

Or a very good example of games you can't really complete would be Everquest,  I would love to see someone review that game without at least dumping a year of their life into it. The review would sound completely n00bish to most of the people who still play on a regular basis unless the person really immersed themself in it. And even then I don't think a year of playing nightly is long enough to take in even half of the content....but MMOs are strange beast so different rules should probably apply.

The reviewer profession has become such a rat race to get the first reviews out in their name that most can't afford to finish a game before publishing a review. It's just a further testament to how arbitrary reviews have become.

Some games are just too large to complete for a single review. Anything over 20 hours is not worth the time to finish for a single review.

Reviewers don't have the time to finish the game before the deadline (their are exceptions though).

Most game review sites do not finish the game, but that doesn't stop them from giving reviews - and scores.

How many times have you told a friend, "ZOMG! [insert game title here] is awesome! You have to check it out!" after spending 1-2 hours with it, barely completing the first level?

Admittedly, reviewers are doling out scores which can positively or negatively impact a reader's opinion about a game, but that's what they're there for, right? As long as the reviewers actually spent some quality time and progressed in the game enough to understand the underlying gameplay mechanics and how the different gameplay elements mesh, that's good enough for me - just like my buddy telling me that [insert game title here] is a must-play or a dud.

/opinion 

Is it a coincidence that all games on Game Rankings appear to be in their correct scores, when compared to one another?

?

Correct scores? Care to explicate?