Ah, you ask good, lucky for I can provide an answer. I think that which makes FF so special is that every game is sooo different, yet their are exactly the same, at the same time. The chocobos, the moogles, the battle system, mostly is untampered with, FF has many devices that make you realize this is Final Fantasy. Although every title has new systematical concepts, a fresh story, and a completely different world.
On top of the that the immerisive and daring characters. Basically, you know when your playing Final Fantasy when you Shout"this is Sparta!!!!!!!! Lolz I mean Final Fantasy.