Everything a JRPG should be, and more.
Story - The story is simply awesome. Each character(even secret ones) have their own side story you have to beat along the way to taking down the evil Shinra company, you start as an eco terrorist trying to save the world but the plot soon changes and gets pretty complicated with all the twists and turns you would expect, and you find out all is not what it seems. The characters are all pretty different and contrast each other, some you will love, some you will hate. The haters call Cloud emo, they have him confused with Squall. Play the game to the third disc to see why Cloud is the way he is, he has reason to be "weird".
Music - The beautiful music perfectly matches the scenes on screen, it makes you sad when you should be, it makes you excited in boss battles, it's one of the best and most memorable scores I've heard in a game. These tunes will stick in your head for days.
Gameplay - OK it's turn based random battles. Some say turn based is antiquated, I still love it, and as far as turn based random battles go, this is pretty much perfection. There's towns to explore and once you get out of Midgar a world map, it's linear but always gives you the impression you have plenty to explore and do. This is one of the main draws of the game, it's not so open you get lost and/or sidetracked like in The Elder Scrolls, or too linear it's like a long tunnel such FFXIII. There's plenty of extras to do including secret characters to recruit and side missions, you should expect to spend 80+ hours if you do everything first time around.
Graphics - The graphics aren't nowhere near as bad as the haters claim. I say when the graphics interfere with gameplay then you have a reason to complain, this is still as playable as the day it was released. The backgrounds still look fine, it's only the characters that look a little clunky.
Flaws. These are hard to find actually. The controls could be better, it could have analogue for example, but analogue wasn't out back then. Being able to pause outside of battle would be nice too.
Overall - I think it's as close to perfection as a game has yet achieved.