Only posting hints here but spoiler tagging it just in case
There are multiple endings depending on interactions with NPCs and several NPC quest lines with different endings in themselves depending on how you do them. Some of them even overlap with different NPCs, people are still figuring out all the possibilities.
There are enemies that turn hostile based on your interactions with other NPCs, there are special interactions with NPCs in the world (think the special invasion in Anor Londo in DS1 and one or two that were huge, awesome surprises.
There is an NPC whose sex depends on the sex of your character.
There are 4 or 5 optional areas. Some of them are very hard to find.
Covenants are more like DS2 than DS1 but there's no rat covenant like silliness. Two of them are well hidden, one really wel hidden that requires some daring experimentation.
All in all I like the DS3 NPCs best, only Demon's Souls is on equal footing there.
That sounds REALLY AWESOME, even better than i expected!
God lord, 7 days...until PC release T_T