Computer graphics: Examples include decals, reflection/wave water effects, environment lighting and dynamic shadows, vertex animation, terrain blending, Fog of War / Shroud of Darkness, skyboxes, vertex/pixel shader material framework, and a particle weather system.
AI programming: Examples include everything from pathfinding, through basic unit AI (chickens running away when attacked, bears attacking spontaneously) to high-level opponent modeling - gathering resources, laying out a city, building an army and more.
Gameplay programming: Implementing the mechanics behind trade, and construction of structures, creating various unit formations, writing random map scripts.