Not counting maps and missions I put things into two cathegories, Art and Function. Art is 3D, texture and sound. Function is code and animation.
Art is important and can add a great deal to help realism and immersion, but Function is the core aspect, because it is not about how things look but how features work and are used by the player.
As example. Most generic way to do pistols is to have the slide unlocking after reloading as a separate animation, in real life, when your life depends on it, you slap the mag in and aim the pistol while unlocking the slide after which you can fire immediately. Two actions in one move, that's realism and if the animator doesn't get it right, even the programmer can't help it.
Same with drawing the sidegun, if your life depends on it, you can unsave the pistol and already fire a couple of rounds even before you brought it to aim. This is realism, this is gameplay. These are the things the animator needs to be aware of and work toward to and the programmer needs to make it being usable comfortable and effective (no unnecesssary delays when pushing keys, etc).
Some guns in INF even push it so far as to run a "bring to hip" animation, than run the "unsave gun" animation, after which I can initiate the "aim gun" action. Gosh, I can chat in the meantime while Tangos rip me apart... realism.