-Microsoft Holographic (HoloLens) ready in 5.5, See the details in the Holographic documentation.
-Improvements to Particle System
-The Animation Window has workflow improvements and significantly improved performance
-Look Dev to ease the process of ensuring parity in materials throughout physically-based projects (currently experimental tool)
-Unity Collaborate open beta provides a simple way for entire teams to save and sync their Unity projects.
-Various performance improvement.
More detail can be found Here