Randomize Scale and Rotation
Combined with the Dynamic Placement Tool and the Duplicate keyboard shortcut (CTRL + D), a world builder can blanket a large area with a single asset without visible repetition.
The Random Scale Toggle, when enabled, will cause any newly placed instances to have a random scale ranging from 80% to 120% of normal scale. The effect will apply to asset instances placed via the Asset Panel and Library.
The Random Rotation Toggle, when enabled, will cause any newly placed instances to have a random rotation about the Y axis. The effect will apply to asset instances placed via the Asset Panel and Library.
Applying to Existing Assets
CTRL + R will apply the currently toggled options for Scale and Rotation to all selected instances.
This is a great way to update existing assets in your world, or to apply the randomization again to assets that have already been randomized.
CTRL + SHIFT + R toggles both Randomize Rotation and Randomize Scale.
Other Options for Randomization
To paint random meshes that don't require collision make sure you check out the Dynamic Detail Brush.
For complicated and more specific usage, check out the Space Ref for examples of Library based script interaction.