Details from the release blog:
Massive lighting improvements
5.6 includes the Progressive Lightmapper preview, which provides immediate feedback when trying out different lighting scenarios, and much faster iteration, compared to the current Enlighten solution. 5.6 also offers Light Modes, which allow for various ways of mixing real time and baked lighting for static and dynamic objects.
Improved graphics performance
GPU Instancing which enables new kinds of effects where many similar objects are needed at a very low performance cost, has been improved with support for Procedural Instancing. And, with the addition of Compute Shaders for Metal, you can now add more details to your games by tapping into the raw power of the chipsets on Apple iOS and macOS.
Vulkan support brings increased speed while reducing driver overhead and CPU workload; this leaves the CPU free to do additional computation or rendering and saves on battery life for mobile platforms.
Massive update to Particle System
5.6 greatly extends the range of particle effects to give more options and control to the user. This update also significantly improves Particle System performance.
New Video player
A new multi-platform video player enables playback of 4k video allowing you to build 360 degrees videos VR experiences.
Navigation system improvements
Improved AI and Pathfinding tools–also referred to as the NavMesh system–expand the possibilities to manipulate multiple nav meshes and agents. And, new tools for procedurally generated or dynamically loaded content enable a whole new set of use cases and gameplay options for character navigation.
New 2D tools and improvements
Unity 5.6 adds a whole set of 2D features that give you more control and make it easier to create complex 2D objects. New features for 2D physics enable new kinds of gameplay and effects, including fully featured particle effects interacting with 2D objects.
One of the top performing tools on the Unity Asset Store is now available for free to users of 5.3+ and will be soon be natively integrated into Unity. TextMesh Pro features advanced text rendering with dynamic visual text styling, along with greatly improved control over text formatting and layout.
Performance Reporting and debugging improvements
In addition to exception reports, Performance Reporting now collects native crashes for iOS. Physics debug visualization and profiler improvements make it easier to find the source of performance issues in the game.
Seamlessly publish to Facebook Gameroom, as well as Google Daydream and Cardboard for Android and iOS. Nintendo Switch support is also available now.
Unity Collaborate (Beta)
We added new options when publishing changes for finer control of your Collaborate projects.
Experimental support for WebAssembly
In 5.6 we bring experimental support for WebAssembly, a new cross-browser technology designed to help improve the Unity WebGL experience.