Freedom, which includes the freedom to mod the game to our liking. This is the best modding community in gaming, because Bethesda was open enough with their game and developers tools, as far back as Morrowind, to allow us all the opportunity of making and playing with mods.
I've read all kinds of reviews about FO4. The critics are largely right, but the point that they are missing is that what really makes these games amazing is still here. It blows my mind how many quality mods are already out, even before the release of the CK. I would have lasted about 8 hours on a console. As long as Bethesda keeps the game open enough, and lets this talented modding community do it's thing, you can count me in, and I'll keep giving them my money.