The Arma 2 engine is very powerful, the maps are huge, the ballistic model is as real as it gets in a game (it's a sim really) the view distance is massive if you're rig can handle it, it simulates a vast amount of military & civilian vehicles, the controls which are often complained about are quite complicated because of the level of control it gives you not just control of you're player but squad commands way beyond what any other game has ever given a player (except maybe some high end flight sims), the mission editor is crazy in what it allows you to do. I challenge anyone to name a game that does all of this better or even as well as Arma 2.
IMO anyone who complains about the engine after only playing DayZ has not even scratched the surface & as such should not be so vocal calling the Arma engine crap, it just make them look like an ignoramus to anyone with a little knowledge of what the engine can do.