A lot has changed in LightWave. Here is a potted list with links to pages that describe the features:
Pages describing features new to LightWave 2018 have the label New
The LightWave scene format LWS has been incremented to take account of LightWave's new features. Previous versions of LightWave will not be able to load LightWave 2018 scenes, you will need to export in a previous version LWS format.
LightWave 2018 now comes with a set of primitive objects in addition to the mesh objects it has always supported. Right now, you can create Primitive Shapes, Volumetric Primitives and OpenVDB objects.
Primitive shapes (Sphere, Cube, Cylinder, Torus, Cone and Plane for now) can now be made by simply adding a null and selecting shape in Object Properties. Parametric shapes can be textured like geometry and have a UV map for texturing built-in. They can also be displaced and, because they aren't based on geometry, can have very fine levels of detail.
HyperVoxels is being reborn as a modern volumetric object with complete integration to the renderer. Where HyperVoxels of old had problems with transparency and shading, 2018's Volumetric Primitives take advantage of all the new renderer goodness.
Items and sequences compiled into OpenVDB containers in other software can be brought into Layout as objects.
Lights have been completely overhauled in 2018 and now have a physical size (and presence in the render if desired).
Falloff is now enabled by default. The Node Editor can be used to create custom falloffs.
Most lights can optionally be visible to the rendering camera (Area, Linear, NGon, Spherical and Spot when larger than 0).
Rather than the abstract percent measurement that LightWave lights used before, they are now measured in Lux (Lux is equal to one lumen per square meter), making it easier to generate accurate real-world lighting. It means that the larger the light, the more brightness it needs unless normalized, and that IES lights should now be exactly the right brightness without further tweaking.
An Environment light is non-localized, non-directional diffused lighting from a backdrop. You can use its intensity to control the brightness of the illumination from your backdrop image or gradient.
Area and NGon lights can be Portals. Portals provide no illumination per se, but concentrate light through openings. You would use them for a doorway or window to bring outside lighting through these openings for the interior.
A new light that uses a primitive object as the basis for its shape. Object geometry or the new Primitives can be used.
The Dome light has been removed and the Distant light improved to allow for an angle setting to provide the same effect. Shadow maps are no more and Spotlights can have soft shadows. Most lights can use images. For the Spotlight, the image will be projected. For other lights, the light color will be tinted by the image.
Light Groups are a way of adding more granular control for one or more lights that you want to have separate access to when compositing. The lights in a group have their own buffers where lights not in a group contribute to the standard buffer output.
With the Volumetric Primitives and OpenVDB already mentioned in Primitive types, there are new options for lights to affect volumetrics, a volumetric scattering effect (for so-called "God rays") and new fog settings.
The Surface Editor has been overhauled to offer ease of use for simple material texturing without needing to enter the node editor, a VPR-based Surface Preview mode that surpasses the previous sample in Surface Editor and new materials.
Because the concentration on nodal surfacing has intensified, often places where once were T buttons have been replaced by a node editor. There is still a layer-based component to the Surface Editor for the standard material, but all others have node-only inputs.
The Principled BSDF material has replaced Standard as the default material used.
In addition, the Surface Editor has a new RMB menu when clicking on a surface name.
Cel shading has received a massive update in LightWave 2018. There are now two cel integrators in the Surface Editor Shading Models. With them, creating consistent shading is made simple with diffuse and specular gradients.
Secondly, Layout's OpenGL has been improved to give a better idea of how cel shading will look without even needing to render or go into VPR.
In addition, the New Surface Shaded switch in Object Properties allows full material selection for edges using the Mesh Element Switch to create more cel shading possibilities, and the new Global Edge Multiplier and Edge Vertical Points settings make rendering cel-shaded edges for different resolution outputs simple.
FiberFX has been recreated as a primitive object. There is no longer a volumetric version.
New nodal motion modifiers can link mesh pivot points to meshes and vertices and Bullet objects including parts.
With the PBRGLSL Shading Method, display in OpenGL matches VPR as closely as possible. The screenshot above shows the Principled BSDF material in OpenGL and VPR side by side. Apart from the shadows evident in VPR, the shading matches closely.
In Layout you can now add additional floating viewports that can be of any type and can be moved onto a different monitor in a multi-monitor setup.
Layout and Modeler's main menus have been rebuilt for better efficiency and new interface elements like clipboard functions have been added to speed workflow.
LightWave's interpolated radiosity rendering has improvements to sampling.
Rather than the lines of previous versions, LightWave's render engine now uses user-sizable tiles.
AOVs are Arbitrary Output Variables, and LightWave now supports rendering of outputs like Direct_Diffuse, Depth and Alpha among many others, directly in VPR and final render.
VPR can not only show the final render as before but now can show specific buffers like Alpha, Depth, Motion and more. It also now acts as a replacement for the old Surface Editor preview using either preset objects or scene objects.
Sometimes you really need to check that one detail that will make or break your render and you don't have the time to wait for the whole thing to finish. Now you can direct where VPR concentrates its firepower with cursor-targeted refinement.
You can now create presets for rendering settings using the F8 presets window.
LightWave is now easily installed and controlled as a renderslave or host with a graphical user interface making controlling a renderfarm simple.
The LightWave object format LWO has been incremented to allow for larger quantities of nodes and surfaces. The LWO3 format won't open in previous versions of LightWave, you will need to either export an LWO2 format object or use the LWO3 loader plugin for previous LightWave versions here.
The UDIM method of assigning images as textures and organizing UV maps was created by WETA for the Lord of the Rings films and has now been implemented for LightWave.
New "Live" tools update the viewport as you change settings in the numeric panel.
More tools now respect the Symmetry state in Modeler - Edit Edges, Lattice, Knife, and BandGlue.
You can now Rotate, Stretch and Scale with more precision using the Right Mouse Button (RMB). In addition, using CTRL will constrain rotations to 15° increments.
Layout View has been added to the View modes present in Modeler. It is an immobile view that exactly replicates the view in Layout. If you change the viewport in Layout, Modeler's will follow suit.
LightWave's documentation is now in the form of the online wiki you are reading. It will always be up-to-date and improvements and changes can be easily made. It is accessed from Layout and Modeler by hitting F1.
Reporting Feature Requests or bugs that need fixing has never been easier. From within LightWave you can send both straight to NewTek.