Operation Flashpoint campaign was much better than ArmA campaign. The mission variety, the length, the little cutscenes, the dialogue between soldiers, the overall narrative, etc.
That said, in ArmA there was a little new feature (a bit hidden) that improved the game a lot. Customizable AI level!. In Operation Flashpoint you could only toggle some realism options, in ArmA (and i suppose also in ArmA 2) you can toggle the same options and also raise or descrease the AI level, and with different settings for AI smartness and AI accuracy.
So you can lower the AI level to more manegable levels (and dont have to repeat 30 times a level), and specifically only lower the goddaamn too accurate AI accuracy.
In ArmA you also can save always, in the same slot, without having to use cheats like in Ofp, where you could only save one time in one slot.
ArmA flaws were a very bland campaign, high requisites, and bugs aplenty (even by Ofp standards!). If they fix these three flaws in ArmA 2, i will be happy.