More than anything this game just makes me happy. Part of it is nostalgia I'm sure, but I played through the game again recently and I like it even more now than I did when I was younger. The characters, the world, the story, everything about it is so charming. That's not to say the game is all sunshine; plenty of tragic shit happens, but because of the characters it feels more like you're on an adventure rather than an arduous journey to save the world like many RPGs. Toward the end the game does become quite literally about saving the earth, which is a little disappointing, but it's done well enough that doesn't hurt the overall feel of the game. Zidane is my favorite of the PSX Final Fantasy protagonists by far. His optimism, humor, and thirst for adventure is largely what makes the game so fun. The romance that develops between him and Garnet is pretty cute also.
The skills system is fairly simple, especially when compared to Final Fantasy VIII before it, but it works very well. Various abilities, such as increased damage against flying enemies, are tied to weapons, armor, and accessories, and when your party kills enough enemies with those items equipped they master those skills and are able to use them in battle. This isn't a case like FFVIII where the abilities system is the one of the main draws for me, but I do like it and it's pretty balanced. What's more fun in this game is obtaining the equipment that contains the skills. There are synth shops where you can combine old weapons and armor to form new, more powerful equipment, and playing the Chocobo Hot & Cold mini-game will earn you some of the best items in the game. I still think the Triple Triad card game in FFVIII is the best mini-game in any of the PSX Final Fantasies, but Chocobo Hot & Cold is close.
Bottom line, this is an incredibly fun game, probably my all-time favorite, and anyone that enjoys RPGs even a little bit should play it.