Define custom brush strokes
Microsoft Expression Design lets you create your own custom brush strokes and apply them as a stroke to any path or shape. A custom brush stroke (which some people call a "skeletal stroke") could be a natural-looking dab of paint that resembles the brush trail left by a bristle brush, or a surrealistically stretched centipede with as many pairs of legs as is allowed by the length of your brush trail. Brush strokes can take any shape and form, and have many variations that you can control.
You can define any vector object (or group of vector objects) or any bitmapped image file as a brush stroke. A simple stroke applied to a path will be stretched uniformly along that path. However, you can make more complex strokes that follow rules, such as "anchor the first and last elements so they don't stretch, and then repeat the center elements as many times as necessary along the path."