I've not been playing this great game for very long so far, so I can't complain about too much... still learning much of it.
But since you asked...
Aircraft: The Helicopters seem to lack any kind of mass.... there is no feeling of momentum or inertia. They shoot into the air at the slightest touch of collective, they change direction on a dime. They also don't seem to sideslip very well... add lateral cyclic, and you are leaning almost 45 degrees to the side before you really start to move. One thing that ALL heli sims seem to get wrong... the fuselage tilts way too much at speed. The fuselage should hang much like a pendulum under the main rotor, it is the rotor disc that tilts to cause direction change and speed, not the entire aircraft.
Maybe mass is an overall vehicle problem... that would explain why tanks bounce about when shot at with rifles.
Still, it's a lot of fun. One of my favorite things to do is just fly around in an OH-58. The visuals of flying just over the treetops, with the truly amazing forests in this game, are just superb... and this is coming from a real-life helicopter pilot.