Seems interesting, what kind of game is ArmA?
Armed Assault is a military sim. You essentially play as a soldier (or pilot, tank crew or civilian) but you are able to jump into cars, trucks, APCs, tanks, chopers, planes, boats & static defences (artillary, MG post, searchlight, etc.) and it's played on a huge scale (400sq km, free-to-roam island).
Single player campaign leaves a lot to be desired, but probably only because the online community have the most talented mission makers I've ever seen the works of. Multiplayer can range from small scale missions, such as CTF, DM, Flag Fight (a DM-CTF with only 1 flag) small C&H and small coops, to missions that incorporate the entire island and everything between. To give an example of mid-scale, there are a load of C&H missions that are quite similar to BF maps (but with a time limit, not death tickets).
My favorite style of mission art the RTS type, where one player builds a base & commands the AI, and the rest capture certain areas to gain income & use that income to buy all the infantry, vehicals, aircraft & boats they can afford, from the base, & battle it out with the other team.
I'll do a search for some online gameplay clips...
Berzerk C&H
Evolution, Epic coop (takes an average of 10hrs to complete)
There's also a
Zombie mod
This one is a DM called Road Rage. I filmed this while I was having a meal break.