From
http://www.thinkythings.org/knotwork/knotwork-general.html
Certain properties of celtic knotwork define its style:
Over-and-under construction: The cords of the knotwork are woven such that cord intersections alternate between "overs" and "unders". In a few cases, you may see two overs or two unders in a row on a pattern that can not be drawn the correct way, but these are rare.
Uniform cord width: The cords of the knotwork design maintain a consistent width. However, if the knotwork makes a transition into some other pattern or decoration, you may see some variation in cord width during the transition.
Pointed returns: When a cord turns back on itself, the turning point is generally spade-shaped, not U-shaped.
Repetition: Knotwork is intended for use along borders or to fill space; therefore, it is a small pattern, repeated over and over. Knotwork that does not follow a repeating pattern tends to look like a bowl of spaghetti.
Single-cord construction: The best examples of knotwork are a single, continuous cord that turns back on itself rather than several intertwined cords. Some patterns require two or more cords, but ideally a single strand should be used.
Common patterns: J. Romilly Allen has identified
eight basic knotwork patterns used to create almost all patterns found in celtic art. Many more patterns are possible, but Allen's eight knots represent the most commonly used.
Space filling: Knotwork is often used to fill up margins or other blank spaces.
Of course, these are general characteristics, not rules. There are plenty of exceptions. However, these features seem to be present in a majority of knotwork examples, and they seem to contribute to the knotwork aesthetic.