1.) Story - Seeing how all the potential plot points end up... Wild Hunt/Ciri, Nilfgaard/Northern Kingdoms, Roche, Iroveth, Sile, Phillipa, Saskia, and generally what has happened during the end of WItcher 2 before Witcher 3 picks up. Hoping we learn a lot more about what went on with Yen when she was held by Nilfgaard.
2.) Exploring - Finding everything interesting there is a to find in the game, including things we know about, ones we don't and ones that might "not be of this world".
3.) Combat - Fighting all the different assortments of monsters, people, etc