I would argue the main problem is that the game doesn't lean enough in any direction. It just picks elements from the shooter genre, MMO, RPG, and such. However, it doesn't really excel at any of them. Yes, the combat is really good, but the encounters and levels just feel so meaningless. The bombast of Call of Duty has the spectable of finely tuned scripted levels, while RPGs feature deep storylines, ample character customization and gear. This has the bare minimum of everything, and it doesn't really work.
The first time I went through Old Russia, I didn't want to go to the cool looking places in the free roam, thinking there would be some cool story related stuff there in the missions and that I didn't want to ruin the experience.
But that never happened.
It amazes me how much work was put into these incredible vistas and locations, yet the missions and story elements themselves are more basic than even Diablo 3. Why not have some audio logs, NPC's - something - anything, just to give these worlds some depth or even context? Even running through a zone in World of Warcraft just oozes with adventure, mystery, a sense of discovery and history.
I think the best way to describe it is like I'm walking through the level in a level editor with all the scripting, NPC's and other elements turned off.