I do think story IS the main drive behind BP's success as Mat, but also it is definitely not the main reason for an FS-like game to succeed at all... gameplay innovation does bring an impressive amount of pluses to the equation.
Anyways, personally I think the thing that made BP so great for me was the story, simply because I wasn't expecting it to be that good, gameplay on the other side... I'm always looking forward a refined product when playing an FS mission/campaign, Hard Light community requires it actually because we're a bunch of crazy perfectionists after all.