It just seems that most other sims are very arcade-ish, compared to this one. Yes, you have the arcade-ish players in AH, but you can't do as many unrealistic things as you can in some other flight sims.
I would venture to say that FSX is modeled fairly well, but their combat flight sim leaves much to be desired.
To be honest, aside from researching the specs and capabilities of any specific warbiird and comparing it against what it can do in-game.
I personally (and I am fairly certain that I can speak for most folks here) have never flown any WWII a/c in real life, so I have nothing to compare it to.
It just seems that, from what I have read about certain a/c compared to how they perform in AH2, this game seems to hit closest to the mark. Maybe not perfect, but it is always evolving, and they are always making improvements.