I really liked Operation Flashpoint. ARMA and even ARMA 2 are the same damn game as OFP. The single player campaigns are just balanced very poorly, and have more of a tacticool "Murrica FUCK YEAH" vibe in their presentation.
In OFP, you lose your entire squad, get shot down in enemy territory, flee on foot through a forest while evading helicopters and patrolling APCs, conduct guerilla warfare as a civilian with barely any official assistance, and a whole lot of other missions and plot developments play up the war as a hellish tragedy that affects civilians most of all.
ARMA, on the other hand, bhas very little of that characterization. ARMA 1, from the little I played, has almost no characters at all besides the narrator, whose name I have already forgotten. The bitter retreat from a terrifyingly large force, so played for drama in OFP, takes up maybe ten seconds of a silhouetted slideshow in this game.
And in ARMA 2, you come upon evidence of civilian slaughter, torture and even rape in the first mission. The tone of these scenes is "Oh. We'll get those bastatds. I mean, I guess." Your squad size is down to four highly-specialized units, from potentially commanding multiple teams of units in OFP. In addition to taticool door-breaching scenes (involving flashbangs that your soldiers don't even have), rock music replacing the orchestral score, and the fact that the US is arriving fully prepared, the tone of the story is more along the lines of a Tom Clancy game than OFP, which has been compared to Saving Private Ryan and Full Metal Jacket.