MGS4 producer Ryan Payton's sent an email to Kotaku explaining in a fair bit of detail the review restriction placed on US reviewers of the game.
From the note:
We asked reviewers to avoid the following topics:
-Length of cutscenes (the ending in particular)
-Number of environments
-(Plus a half dozen story-specific items)
The game requires an eight minute install, as well as a number of two to three-minute installs between acts.
As for the cutscenes, reviewers are more than happy to comment on whether they're too long or short. We simply want reviewers to refrain from describing which scenes are long, thus spoiling some of the experience because players will know what to expect when a scene is unfolding.
I want to make it clear that, from today, reviewers are welcome to discuss the length of the cutscenes and install times, but we ask that they not get too specific about the cinematic times and what happens in later install sequences...
Well worth reading. It does clarify a lot of the issues that blew up surrounding this last week.