BrightCandle put it perfectly.
ArmA II is a flawed, unpolished game with plenty of bugs and quirks that keep it out of mainstream popularity, not even taking into account the steep learning curve and realism.
Even after patches and some mods, it has some quirky AI issues, graphical glitches, and other bugs.
HOWEVER,
There's also no game like it on the market today. What you can do, drive, fly, man, or otherwise control, across the massive, populated map, is unrivaled. It simply is one of the most impressively ambitious games out there, in any genre. When playing cooperatively with real people who are devoted to the mission, there aren't many games out there that can top it. I am constantly amazed at the sheer scope and scale of EVERYTHING that's going on in the world, and how realistic it still manages to be.
ArmA is the kind of game you have to really WANT to get into. It's too flawed and complicated coming from your standard military FPS these days to attract people who are just looking for another game to try. The style and breadth of ArmA has to really appeal to you, and you have to able to forgive its quirks (and also learn to download a few mods and user missions!).
But once you do, it's close to a masterpiece. Nothing really comes close to touching it.