I happen to enjoy the defined ending. Most stories that leave the world open to exploration at the end have no contunity with the actions of the story. In fable 3, "horray! the king who beat the uber evil is buying at my shop", that's it. No "holy freaking crap, it's the king who saved six million of us, let's atleast offer him a nice cup of tea." Even Broken Steel didn't fully correct to massive perception shift that should've occured with the world's NPC. "He saved the wastes and almost died to give us free water, you think he could haggle with these guys in power armor for us?" The story is epic and, to remain so, needs it's D-Day to cap off the epic struggle. After saving the world, do you really want to see your character wandering into every single rathole and random shack looking for crumbs of new story fluff missed in the main game. If you've played games with no defined ending, then you've probably realized that those games are fun for about 1 hour after you've won, then you go back to Fallout 3 or NV to try a Legion/melee build.