As Kurt Zisa said it would be most likely towards the characters and story. Perhaps the job system being not as open as in FFV, which more players probably played before FFIII, is a reason they dislike the game.

I liked the game. Had a pure fantasy and old-school feel. A simpler job system but I was experimental nonetheless.

Final Fantasy III was made for the NES which some people may forget. For its time it was pretty good. The DS version upgrades the characters and story a bit, but it keeps the old-school feel. Nothing overly complicated or in-depth.