Korra allowed the spirits back in the human world and that felt like a good enough ending. i felt like they couldn't do anymore to top that off, but i was wrong. i'm looking forward to seeing how this all turns off.
but seriously, i saw it like this: Aang, being an airbender, had to master the other three elements. he does so, defeats the firelord; then Korra comes along, her being a waterbender, she mastered all elements except for air. which felt clever to me. "Last airbender" book 1 water, book 2 earth, book 3 fire; then Legend of Korra: Book 1 air. Book 2 spirits. it felt like it was over to me.