Given the changes in each individual FF game, I don't think it reasonable to say that there is any particular defining thing about it. How can you compare them? They are, honestly, stand-alone games with a few commonalities, a sometimes similar battle system, and the same name. With the exception of things like DoC and Biggs and Wedge and some enemies, each one is pretty distinct.

As an aside I was totally in love with this game when I heard the callsigns were Antlion and Tonberry.