I think was a combination of a little of everything.
Mysterious character.
Loose ends with Lucrecia and Hojo.
Good story line to build on.
In either case I'm glad they made DoC, the game is decent, would be great if it wasn't for the way the camera moved, but more than anything it adds to the FF7 storyline tremendously. I always felt Hojo and Lucrecia were way underplayed in FF7.
Speaking of Vincent, anyone else saw the similarities between Vincent and the dude from the anime series? (FF unlimited I believe).