RULES!
I'm just now playing the original for the first time (never played any PS game, including PSO), and its really really good.
It really lets you roam around a lot and get into trouble very early on, rather than forcing you on a linear path. The "3D" dungeons are incredible for a MS game!
(I'm playing the GBA version, but I assume the master system original was the same)
It seems so much more advanced than Final Fantasy I.