Alright: FF7 had way better Limit Breaks, more optional bosses (which were wicked hard), you could customize your characters, anyone could summon, there were optional characters, the setting had some cool modern stuff (motorcycles, cars, cannons, Midgar, etc.), the main enemy seemed eviler than in IX (Sephiroth>Kuja), and you could get attacks/skills that did more than 9999 damage.
Don't get me wrong, I love FFIX, but when I played FFVII and FFVIII, I realized how much cool stuff that there should have been in the game. Most dissappointing was the lack of a rapid-slashing limit break for the main character like in VII, VII, and X. Sure there was Hades and Ozma to beat up on, but they pale in comparison to Ruby and some of the weapons.
But, the best thing (I think) about IX, was how you could actually 'learn' skills and equip them to have permanent effects in battle. ((The auto-haste+auto-regen combo was awesome)) I just liked being able to be immune to poison and such without having to change equipment. Oh, and I also liked how you could equip five items instead of only two or three.
Well anyway, that's what I think.