(I haven't played it in years so there may be some inaccuracies while my mind blends together every RPG I've ever played, but I'll try to describe why I rate it so highly.)
The best way I can describe it is that it took together the gameplay elements I enjoyed from the RPGs of the previous generation, added a few more and wrapped the entire thing up in an overall package which appealed strongly to my personal tastes.
Regarding gameplay design I really liked the materia system for handling skills and magic etc. I would have liked to have seen this developed more in future games in a different way, the way it was handled in the following games seemed like a step backwards in my opinion, particularly the 'draw magic' stuff used in 8.
Difficulty and pacing was tuned well I thought and I didn't really find it all that grindy except for when going for the optional bosses which were tuned so so tightly, it felt like an amazing achievement to kill them.
I know people have mixed feelings about this and will probably argue that it was shit but I also got pulled into the plot and the character development. I don't get into games on that level so much now, probably because I'm older but maybe also because I didn't have the internet spoiling key plot points 30 minutes after the game was released back then.
I thought the atmosphere was great, graphically there were elements which may not have been amazing, particularly character models but it didn't ruin it for me personally and the soundtrack was top notch.
It may not have aged all that gracefully, and I have no doubts it's not to everyone's taste. Random battles were sort of out of control, summons took half an hour and looking back it did have many flaws - but for what I was looking for in gaming at that point and compared to console RPGs which came before it, it delivered the perfect experience. I won't argue with anyone who dislikes it, popular games will always conjure divisive opinions but I'll always consider it one of my favourite games.