Old BF2 player here, now switched over to ArmA.
Comparing the two games really is not fair to either. Even with PR, BF2 has a lot of issues with map size and the game engine. ArmA is complex with a significant learning curve.
My favorite missions are Domination and Evolution, both COOPs. The need for good teamwork will make or break a game in ArmA. If you hop onto a server loaded with retarded players it's going to be miserable. So find a good group of players and hang with them as much as you can.
There are a lot of add-ons and mods to tinker with too. I'm biased here, but ACE is one mod you should certainly concider. Very similar to PR for BF2 in that it aims to enhance a lot of the ArmA environment - though without the odd rule changes PR added.