Some of the highlights:

The New Sky Model has better overall tint to the lighting in daytime, and supports the sun being below the horizon for dawn/twilight effects
New Lens Effects give you vastly more control over bloom & glare
78 new materials in the Corona Material Library, including new Flooring, Carpet and Ceramic Tiles categories
Avoid texture repetitions with updates to the UvwRandomizer
New Adaptive Environment Sampler removes the need for Portals, saving you the time (and pain!) of setting those up, while giving more accurate results
Support for foam in Phoenix FD
And more!