Whenever you're playing Halo 4, Assassin's Creed or just anything else in general, there is always a sense of bulk. What do I mean; I mean that whenever you come out of that real time sequence where the player looked loose and like a ragdoll, a rush of bulk kicks in and the player looks like a robot when you gain control. Sure, it is really hard to make interactional controls for the player, but we live in the HD era of games, where the edges of textures are unknown, and lighting is a mechanism, rather than a manual placement. Another thing that doesn't make games look real: single sequence actions. If we literally reloaded our M-16 the same exact way every time, that we would just look like robots. The animation should be several, but all of them should be within different time spans as well. When is the last time you reloaded in two seconds twice in one day? Exactly. Speaking of angles...
When the first person perspective, The weapon is visible in a mode called "hip-fire" but unless you have a height disorder, there is no way you should even see the tip of the barrel from your hip side. Hold a weapon at where the weapon usually sits for a long time is no joke. You're carrying weapons from 8 - 100 pounds, and even then, they act like you were one of the lucky ones at boot camp. I know, I pushing this a little too heavy on the Call of Duty example, but who can blame me? It is most noticeable in Call of Duty!
Weapon swapping; unless you have four arms, there's no way you can swap weapons like you do in modern games. It always looks like the weapon swinging out of view and another swinging in. Wouldn't you look at your gun while you are holstering it so you would know that you did it correctly? I'm not gonna even start with reloading...
Melee; games need stamina bars. Moving your hands to cause sharp or blunt bodily harm is not an easy task to perform. Knifes are really, really hard to remove once stabbed into something. You never see anyone using the butt of the rifle with that metal butt specifically designed for cqc...
In conclusion, these are just few of the reasons why FPS games in general are flat out unrealistic.