Sometimes being tied to a franchise isn't a good thing, especially when the franchise has been put out several incredibly great games in a row. People begin to have expectations, they begin to believe they know what the game should be about and have trouble when the game doesn't necessarily follow their beliefs.
Critics aren't immune to these problems and preconceived notions, as much as we may wish we were. Take Metal Gear Rising: Revengeance (and ignore its mouthful of a title), both in terms of story and gameplay the game has a whole lot to live up to following the four home console Metal Gear Solid games which preceded it. Revengeance may take place in the same world and may have Raiden as its central character but it feels as though it may have done better outside the franchise rather than as a part of it.
It is an exceptionally good title, but comparisons don't help it. Those games are "tactical espionage action" Revengeance is an out-and-out action title and anyone expecting something different (an expectation that is not unfair) is going to be sorely disappointed.
The story takes place within the same world and is an outgrowth of what we've already seen occur as a result of Solid Snake (and Raiden) doing his thing, but there is no reason it necessarily has to be a part of it. In fact, all that would have to happen to make Revengeance an entirely separate game is to not use acronyms like PMCs and not refer to The Patriots (or any sons thereof) and generally not wink at Solid fans. In brief (because I hate spoiling the plot of any game), you're going to go and hunt down some terrorists who are doing pretty nefarious things.
In terms of gameplay, there really isn't much here that follows from any of the solid titles. So much of those games made the player concerned with sneaking, and that is far less at play here. Guards do still become alerted to your presence in the same way they did previously (question marks and exclamation points above their heads), but it feels less important if much of the time there's no reason to sneak (stealth kills of strong enemies are still pretty cool however).