Real Time Shadows using an adjustable shadow map resolution and a Random Rotation Kernel for smoother, less aliased edges.
On top of this, there are coloured shadows from translucent objects such as stained glass!
Raytraced Screen Space Reflections for dynamic reflections off the environment off objects such as water and other blocks (if specular map is supplied by the resource pack).
Water refraction (also visible through translucent blocks) gives a distorted image, just like in real life!
Steep Parallax Mapping gives the illusion of depth in blocks beyond the capabilities of standard Normal Maps.
High Quality Bloom gives the impression of light bleeding to simulate a higher luminance range.
Highly optimized Atmoshperic Scattering for a realistic skybox. Adapted from here. Temp image supplied by Chroma Shaders (thanks to my good friend Sergeant Sarcasm)
High quality Volumetric Lighting, a vastly superious method of Godrays over the traditional Screen-Space method, for beautiful light shafts.
A choice between truly Volumetric Clouds or Noise-Based 2D Clouds, or both!
Waving Foilage and Flora to simulate wind, becoming stronger during storms.