Holy shit... I just finished this game and that's all I can say about it.
There's something completely satisfying about some of the close-quarters attacks. I still smile every time I elbow an unsuspecting enemy in the face, shoot him in the knee and plant one between his eyes as he's falling to the ground.
The interactive interrogation scenes were awesome as well... I liked it better than just having regular cut scenes--keeps you involved in the story a bit more.
Being able to upgrade your weapons a bit was a nice touch too. The ability to mark and execute was also very cool and made everything feel much more spy like. I found myself spending a long time setting up something that would take only a few seconds but be completely badass.
i said it once, and i'l say it again. splinter cell is the best game series in history