The key feature is Solaris, a layout look-dev and lighting platform which is USD compliant and can utilise Pixar’s Hydra. This announcement was made at SIGGRAPH 2019 and might not mean a lot to most people but it will soon become a big thing in VFX.
Houdini 18 also brings with it a brand new renderer designed replace Mantra under the name of Karma. Mantra will still exist but Karma will be the faster renderer with an emphasis on IPR and Solaris integration. GPU rendering is in beta stage and will be released in a later version.
Alongside the two big features you’ll find improvements in most of the other fantastic tools Houdini has to offer. The PDG and Vellum features that were new in 17.5 get updates along with all the other tools such as Pyro, Fluids, Dynamics, and Destruction.
SideFX continue to develop faster than ever before, evolving tools continuously. They’ve also recently updated the Game Development Toolset to SideFX Labs to include all extra tools in a more manageable format.