The main reason this doesn't get a 4 or 5 from me has to do with how it didn't live up to its potential at all. While some sequences were amazing, and while the environments and main game mechanics were all fine, the fact is that this never really seemed to achieve greatness.
One moment in particular really demonstrated this for me: a cutscene shows you setting off the alarm inside a building, and you then have a very very limited amount of time to run through a set of doors before they close. You have no time to take cover, yet you must shoot enemies who can kill you in two or three shots. The checkpoint is before the cutscene, and I had to redo this part about 10 or 15 times, over the course of a few days because the frustration was too much to bear. While there are no other such atrociously designed sections in the game, most of the second half is comprised of needlessly hard sections, that forgo the originality that was supposed to save the Splinter Cell series in exchange for uninspired government buildings and japanese-like gameplay design. Taking away all of the awesome new cover mechanics and throwing tons of enemies at you just struck me as a poor way to extend an already ridiculously short game. It's really quite frustrating, as some other sections are quite excellent, notably the one that takes place in a Carnival, and one that takes place at the Lincoln memorial.
As far as story is concerned, it's standard fare, though it is also very poorly explained, expecting you to memorize everyone's name and face after one brief mention, and expecting everyone to have completed the last Splinter Cell game. There are also so many twists that you just stop being surprised by any of them by the end. Though by then it doesn't really matter, since you're just half-heartedly rushing through the levels without bothering to hide, anyway, because all the fun has been sucked out of it.
In a sentence:
While there are a few great levels, this game was a huge waste of potential due to moronic difficulty levels, misuse of the new mechanics (why take them away?!) and an unengaging story.