Custom item textures.
Better Glass. More transparency options for glass textures.
Better Skies. Add your own textures to the day and night skies. Many options for fading and blending.
Custom Colors mod. Texture pack authors can now customize many colors in the game including swamps, lighting, and more. See Misa's texture pack for examples.
Balthichou's RandomMobs mod.
Extended animation, mipmapping, and font support.
Ability to load other mods like ModLoader. See "Installing other Mods" for details.
Ability to save and load mod profiles and switch between them easily. Also organize your mods by the Minecraft version they go with.