The quality of a down filled product is determined by the fill power. A good product will contain an amount of at least 550.
Thread count, or the number of threads per square inch, is an important detail. A higher count means a softer product that will be more durable and last longer.
Baffling pertains to the type of construction that keeps the down from shifting and bunching. The best blankets will be baffled.
Down comforters can not be washed at home, as they are too large to be efficiently cleaned and may be damaged by the agitator in a home washing machine. Taking it to the dry cleaner is the best way to care for a down comforter.
Down comforters may include warranties that range from a few months to a lifetime. The better products will always be backed by extensive warranties.