Definitely story. The mechanics are fully introduced and integrated into the experience by hour 8 or even before depending on how fast you "get it". The game doesn't evolve beyond that.
They throw in some new weapons here and there, but in the end you will have a set routine of taking on stuff and will probably stick to that (at least I did, unless I was forced into a corner due to not enough ammo, and had to resort to using weapons I was not comformtable with).
But I am happy that ND didn't try to force in some one off gimmick gameplay sections just to freshn up the experience, they have a really solid gameplay loop and they stick to it. For me I was a bit worn out by it in the end, but I doubt that will be the consencus, due to people have different playstyles and the game is surprsingly adapt at supporting them.