Have you played Project Reality? PR plays like a normal FPS but have the teamwork aspect of milsim shooters, the key to the game is that you jump in on any server and get good military style gameplay and communication with random people and very little setup, the teamwork aspect just comes naturally to everyone who plays. You have combined arms gameplay where the maps are sized accordingly, they're big enough for all types of warfare to matter but small enough for there to be good balance between action and silent moments.
With ArmA you have tons of different mods and game modes and will have to search for specific military teamwork style gameplay and communities to play with, with PR that's all there is and you don't have to learn some odd control scheme because it controls like a Battlefield game.
It really is hard to explain, for most people PR and Squad will look like Insurgency or ArmA but you really have to play them to understand.
