Cut or Loop-Is that the question?
The structure of a rug, cut pile, loop pile or a combination, along with the denier of the yarns and a number of other factors will effect whether a rug feels formal, casual or inbetween.
A loop carpet usually has a casual feel as shown above.
By combining cut and loop within the same rug but in the same color you create a tonal pattern in a rug. A subtle pattern is a wonderful touch in a rug.
Cut piles in a washed wool rug will have an informal appearance, particularly when combined with a larger denier of yarn.
Cut pile and a fine denier yarn creates a formal texture. Combine with pattern or a different color palette and create a different look.