Two particular things that stood out for me (which are kind of related) are the length (30 hour story with some delving in sidequests) which didn't make me feel like I was wading slowly through a grind just to get the next dose of story (or to put it another way, it was a very comfortable length for a JRPG's main story) and the fact that so much of the side content was actually hidden.
I never really felt like I was being spoonfed information on sidequests, and it was up to me to actually put the effort in, except for Fort Condor because who didn't go there? :P I went through the game and still felt like I hadn't scratched the surface, even outside of the Gold Saucer there were extra areas, the Weapon bosses, a bonus dungeon (I think), some more backstory with Zack (I think), more Limit Breaks and Materia to get, Chocobo Breeding...
I know you can finish the game without Vincent (I did, and once I did so it hit me what I'd missed), and...I -think- you -have- to get Yuffie by virtue of needing to go to Wutai, I can't remember if Wutai is on the main path, but whatever. The point is the fact it has optional sidequests to get entire CHARACTERS, and in Vincent's case it isn't glaringly obvious that's your reward. That's a real rarity in FF, the only one that instantly comes to mind to do the same is FF Tactics.
Fast-forward to 13 and it's "Oh, hey. Yeah, you know how most of the sidequests in the last game were hunts? Well, now it's all hunts and grinding, in one area, here's how you do them."
There are a lot of things, but the primary reason was that I found the majority of the content to be fun. That, and I enjoyed the overall design of the game. It has an openness to it with regard to content access, and an enjoyable pacing when it comes to introducing new content to explore.
If you're not going to share with the entire class, then don't bother sharing at all. Online is the worst thing to happen to gaming.
The overall epic scale of the game, that had you travel the world and visit everything. Sure, a bunch of games have this, but FFVII was the first one to really give me the feeling of a living, breathing, fully built world.
You're not the official anything of any board. No-one cares about your PC specs either.