The live demo I watched had Geralt, the titular Witcher, running with two companions, one from the previous game, towards a small town known as Flotsam. Along the way, he got into battles, showcasing the new combat system, which again scraps what the previous game used. The demo player told me of how they wanted a more streamlined combat system, making it possible to string together combos that used both quick and strong attacks. This means not having to pause your attacks to switch your stance. Magic also still makes an appearance, though the only spell that was showcased was a stun trap that was set off when enemies ran into it, exploding and stunning all hostiles around it.
After the combat (what later included a boss, which was at least twenty times as large as Geralt was), we were treated to a dialogue moment where Geralt had to convince a set of guards as well as a crowd to let a few people charged with various crimes free. The demoer managed to get the crowd on our side, setting our friend Dandelion free (who fans of the book series might recognize), all while only giving the guards a few bruises.
What does return is the element of choice, allowing you to shape the story as you go. The developer told us that there would be three endings to the story, but a good more number of choices than that to make as you play. In addition, when you reach events that have been shaped by the choices you've made in the past, Geralt will reminisce about what had made things end up this way.
Other things have been improved, though not really finalized so we weren't able to get a look at them: character customization is still a large priority in the game, allowing you to build a Geralt the way you want to. Other game elements such have alchemy have also made a return, but have been streamlined without being simplified.