During the showdown with the Demon, Sweet, Dawn says to Buffy "The hardest thing in this world is to live in it," she is repeating Buffy's last words to her before Buffy sacrificed herself to save Dawn.
The plaque for Coulson mentions that he "gave us his life (a couple of times)." This is reminiscint of the tombstone for Buffy which read that "she saved the world (a lot)"