The game will feature detective-style investigation of monster "crime scenes". Geralt will need to discover which monster from his bestiary is responsible for murdering townsfolk, before he can track it down to its lair and slay it.
There will be a greater emphasis on preparation before monster battles. The developers imply that there will be multiple ways to prepare for each monster, one of them being learning its internal anatomical details (which kinda reminds me of the alien autopsies in X-COM). Another way in which The Witcher 3 will be returning to the roots of the Witcher concept is that monster hunting will be your primary source of income, and also your primary source of crafting materials and mutagens. There will be unique "boss" versions of monsters in the game, like in the original Witcher. The developers are quick to reassure that there won't be any arcade-style "epic" bosses like Witcher 2's infamous Kayran, though.