Modelling:
Hard Surface
Organic
Terrain
Scene Assembly (USD, Universal Scene Description)

Character:
Rigging
Keyframe Animation
Muscles Simulation
Motion Capture & Retargetting

Simulation:
Pyro FX (Fire & Smoke)
Fluids
Fluids / Large Scale (ocean, whitewater)
Destruction FX
Sand, Snow
Hair and Fur
Crowds
Vellum (clothes, softbody)
Abstract FX (magic)

Game development:
Procedural Environments & Assets
Simulations

Rendering:
Mantra / Karma
3rd party (Redshift, Arnold, Renderman)

Other areas:
Stage Lighting
Compositing
Shader Building
Houdini Development Kit
Pipeline Development (Python, PDG)
USD, Universal Scene Description