So far I don't think there are any "must have"s for ArmA other than the 1.08 patch. Lots of stuff in development, though.
MP is the highlight of the game because the friendly AI can be real frustratingly stupid at times.
The SP missions that came with ArmA aren't the best, but you can go to sites like armaholic.com and grab many more that will work SP or MP as well as SP-only missions that put the stock ones to shame. The size of the islands and the terrain diversity and detailed towns makes for some great gaming.
Games like DoD, CS, and BF are more about team v team adversarial, but ArmA/OFP is all about coop. A small human force vs a large AI force plus numerous vehicles. It's not BF type vehicles, though.