If you're reading this review as an ArmA2/ArmA/OFP fan then you already know why you like it, but for people new to the series I thought I'd share an insight as to what makes this game series unique among the genre.
The main point I wish to make about this game is that it is not player-centric. By that I mean that the ingame opponents do not all exist merely to kill you the player, they exist for their own reasons and have their own goals. If you happen to get yourself embroiled in their fight - then they will target you naturally, but you are no more (or less) a target to them than is your AI squad member. You are part of a larger situation, and can influence the outcome, but you have no more priority than any other entity in the game. Contrast this with CoD/MoH etc where the gameplay is triggered by your progress and positions, and where enemy exist only to impede your, and only your, progress. Coupled with the literally open-ended nature of the massive gameplay area, that you are free to achieve any goal you wish, any way that you wish.
In the game, if you were to place two opposing soldiers in the map 10 kilometers away from you, they will proceed to stalk, engage and battle each other in every detail as though you were there to witness it, even though you are too far away to see, hear or influence the situation. A lot of readers might now be asking "what's the point of simulating something the player might never see", but others might see the true gold in this, that the battlefield has a true fidelity unseen in any other FPS tactical shooter. You as the player are NOT the major entity in this way, just as any soldier is not the major entity in any war. Away from your perceptions, AI soldiers will be battling against enemy, boarding helicopters, and moving to new locations and it may, or may not impact on your own gameplay somehow when they reach (or not) their destination.
Each soldier in the game has it's own micro-AI routines that help it to survive as best it can, while attempting to achieve their own ingame goals. This naturally leads to conflict and there you find yourself, in the middle of something larger than yourself. All weapons, vehicles, locations and buildings are open for use at any time, by all players (in the case of multiplay) and AI. The possibilities in a game such as this are limitless, constrained only by your own efforts and imagination.
In truth, I'm speaking mainly about the ingame editor gameplay, not the included single-player campaign. I have not tried the single player campaign (I think of the game as a sandbox rather than a story-led game) but I understand it's not too bad in its own right. But it is remarkably simple to open the editor, place down some units, set various waypoints or goals, and simply let the action begin. Elements can be randomised for extra variance, so that even you as the mission creator do not know where the enemy is or even how many of them there are. With a little finesse and experience, you can generate missions that will keep you entertained for many hours, over many replays, and just keep on making more and more.
So, this is not in essence the same as CoD or MoH or any of the other military shooters, those guys have their strengths and small levels etc, but this game has the scale and flexibility to entertain you for as long as you wish to play it, however long that is.