2. Severed Connection To Past Avatar Lives
Korra's connection to the past Avatars was destroyed following the Harmonic Convergence. This is a huge change, bigger than the Book 1 finale because in the end she was able to regain her bending through her Spiritual link to her past lives, however now that link has been severed. And according to DiMartino:
"We thought this would be a very powerful way to have her in a very dark place where she is really all alone now. She doesn't have her past lives, she doesn't have her Raava, she doesn't have her Avatar spirit with her, and what is she gonna do when she's faced with that? It also just seemed like Korra's whole journey was on a spiritual path and finding out who she truly is and it seemed better that she can't rely on the past Avatars because they're not there anymore. It's a chance for her to move forward and create her own future and destiny."
This will be a huge learning curve for Korra, who'd only just gotten the hang of her Spiritual side. However I'm excited about this turn of events, we'll see Korra struggle through an existential crisis. Who exactly is she without her past lives/Avatars and spiritual guides?