Good answer...
I think it's mostly two things, at least with FFVII.
Firstly, the depth of the storyline. Not only is it original, interesting, and even a bit confusing, but its presented in an intriguing way that keeps your attention. Never is there a deep scene with lots of tedious dialogue, but information is given in an interactive way. For example, the observatory scenes. I guess some might find it a bit tedious, but at least the first time the FMV is pretty and the movement, the action, the elevator makes you a part of it. The only scene that pushed it, I would say, is the mandatory recap (whether you select to or not) at Ghost Square in the Gold Saucer. But even that was helpful, as the story was starting to get confusing. And the lifestream scene in Cloud's conscious--even if it was a pretty linear path, you were still leading Tifa from place to place, slowly unraveling the mystery.
Secondly, the variety. Not specifically fighting system or the world map but all of it in general. How many different spells (including levels, 1,2,3) complete with animation, programming, damage, and effects, are there? How many games contain dozens of different weapons, with in-battle models, for nine different characters, as well as half a hundred pieces of armor and accessories? Its really staggering how much detail went into the making of this game.
Oh, and, of course, Sephiroth and Cloud. Find me two more badass rivals. Really.
