Back when I was a kid I'd play FF2/4 over and over. Two things were different back then though. For one, not many rpgs came out. And if one did I probably wouldn't be getting my hands on it until christmas. If I were lucky blockbuster might have one, but their selection was downright terrible back then. Not that it's particularly great now(Uh..no DS section? I could kinda see that honestly, but then they carry psp games?), but it's like a goldmine compared to what it used to be.
For another, when you're younger you're more creative. I'd just be running from point A to B on the world map for the fifth and in my head I might imagine something like say, a conversation between Yang and Cecil. Being that I'm now 29 instead of 13 I won't just be able to satisfy myself with content that I just made up in my head.
Almost no RPG that comes out is designed with replay in mind and a new average rpg will be more fun than replaying a great RPG. There's newgame+ type things pretty often, but these actually turn me off more than want me to play again. Putting me at the start of the game at level 99 isn't going to be fun for me. In fact it's going to be incredibly boring.
A game needs forks in the plot and character customization to make me want to replay the game now. Things like the sphere grid or liscence board touch on the character customization, but get it wrong in that you eventually get everything. A fork in the road should lead to a choice that leads to vastly different characters. What you do instead is just go down both roads and the customization becomes trivial. Not too many games have that seiken densetsu 3 'play me again, but in a different way' thing going for them.