Welcome back!
You'll find that everything from how planes handle all the way down to gunnery itself has changed. Over the years the airflow code was revamped, as well as how bullets hit planes (not sure of the exact details on that one, just the noticable results -- it felt like there was a "hit bubble" before, and now it doesn't, so you have to get in closer to get hits and land kills).
It really is one of the best games available, even after more than a decade! It's amazing to think of it in that respect.