Rocket - It's a difficult area to define but it is a subject I have discussed at great length with programmers, designers, etc... across multiple projects including when I was on Arma3 as well as DayZ. It's a discussing that brings up passionate arguments for all sides and I can assure you that at Bohemia there are hugely passionate people and it's often discussed.

Consider a very fluid game like Counter-strike. Why is it so fluid? Why does Payday feel very fluid? There are many reasons for this, I will discuss a few and their impact:

Different animations for client and everyone else.

Take for example Payday. Your character displays different animations for others characters than what you see in first person. Your eyes are not located in the camera's location. Your weapon is not in the same position on your screen as it is when on the animations of others. Arma has its basis in realism so there is a very core focus on matching everything. This makes an enormous difficult task of placing enough of the weapon in your first person view to see what you are holding, without making you look crazy/weird in third person. This affects responsiveness and what we can do.

Response to mouse movement is greatly increased in reaction-shooter games like Counterstrike.

That's why they're called reaction/twitch shooters. When you move your mouse your character goes there. Often no head-bob and very fast responsiveness.

I, personally, despise this in video games. I want to take all the designers who design warfare games who do this out for a few weeks in the field. Now, I may have been a terrible shot (and believe me, I was), and my eyesight is not amazing. But I was not alone in the military in that when I was moving, I could not see a damn thing. If I had to describe my body as something during fire and movement I would use words like "uncoordinated, janky". I am wearing body armor, webbing, ammunition, weapon, sidearm, kneepads, helmet, night-vision, maps, compass, waterbottles/camel-back, PRR, goggles, maybe an M72 or an M203. After loading up for a patrol, I often considered the fact I could move as defying the law of physics.

There are plenty of video games that do twitch shooters. Why don't we have one that does not do this, something different? Why don't we explore that and see if there is an answer that doesn't involve instant responses and relies on who is best in the traditional FPS. The problem we do face is that yes, often in the "real world" the delay does not seem as long between deciding to do something and doing it. I discussed this with a psychologist friend who suggested that this might be because in "real life" you feel the movement so you get a sense it is happening, that action is underway. When you decide you want to do something in DayZ, you must wait for it to happen.

Animation transitions

We have actually reintroduced some of these that we took out before. This is because the level of immersion and even the "clunkiness" of it was actually something that defined the series. It caused some delay in doing movements that, in real life, would take some time. There is constant work on improving these - but it is impossible to know if a transition is truly right until it has been tested for many hours. I suspect our animation team will continue their diligent work on this for many years.

Player Controller

Not controls, controller. This is the system the engine uses to convert your key presses into movements of the player. The current system is quite dated, and it has been reworked many times - perhaps a few too many. So we're redoing it from scratch. Please note: this is not player controls (which we already redid, improving the mouse movement greatly) - but the method the engine uses to deal with things.

Conclusion

So in part I agree with the problems you raise - but I do believe that your definition of "fluid/responsive" is not the same as mine. It is not called "firemovement" in the army, it is called "fire and movement". This is because you cannot be effective firing and moving at the same time. My theory is because the human body is clunky and janky, especially when loaded with a bunch of crap.