These have probably been mentioned before but I'll bring it up just incase.
Two bugs which should be fixed for ArmA2:
1) Soldiers in AI groups getting stuck. In ArmA you sometimes have a few of the solders in a group not moving while the rest of the group continues along the waypoint. Never happened in OFP. Very annoying.
2) Helicopters crashing. If a helicopter goes into combat mode after sighting an enemy it almost always flys low and fast and then crashes into a hill, tree or building. Again this very rarely happened in OFP.