We tend to think that the basic way that each first person shooter (FPS) operates is the same, however the style has changed dynamically, over the course of it's lifetime. We can notice this the most in more recent games, where graphics are extremely realistic, which helps you get "into the game" and feel as though you are the character shooting a bunch of random innocents. However the graphics changes is the least noticeable aspect of their entire evolution. Noticeably in recent games, idle animations and gameplay changes have made a dramatic effect on how the gameplay works and feels. For example, idle animations for standing or sitting, whatever you are doing, whether you are scoped or not (however it is more noticable if you are scoped), the characters breathing makes him/her move slightly, thus changing the crosshairs location ever so slightly. This tends to have an effect when you are trying to shoot someone thousands of feet away. Going back in time a bit, sprinting is also another thing that was not always in FPS games, but it had a great basic concept, kudos to the developer who thought of that first. Being able to get around a map quicker, at the expense of aim being inaccurate (or not being able to use your weapon at all while sprinting) was ingenious. It is now common in any kind of shooting game, first or third person. Recoil, of course, was another bright idea. No real explanation is needed here, it just works. Guns have it, so any FPS should as well. First person shooters is one of the few types of games in any genre that requires a game to be entirely 3D. That is, having height, width, and depth, it doesn't matter if the graphics look pixelated, if it has those things it is 3D. Well then again, if you think you can make a 2D FPS, I would love to see you try, although it may involve forgetting the laws of physics. Think about it, the next time you shoot a zillion zombies in Left 4 Dead, or "Accidentally" shoot your ally in Call of Duty.