Read the leaked GI article. Its open world, monster hunting is now a fully fleshed out series of quests, quests come in 3 varieties (Geralt Story/Side Stories/Political Story) each can be done whenever you feel like or not at all, no more QTEs, revamped combat (reading about a monsters weak spots can give you new combat opportunities against said monsters), takes 20-30 minutes to cross entire map (on horseback), world is 20% bigger than Skyrim, even more hair/clothing/armor customization options, etc... Also its the FINAL Witcher game.