What is the main feature that differentiates marsupials from other mammals?

The main feature that distinguishes marsupials from other mammals is the pouch used to carry their young. In fact, the name "marsupial" refers to the name for this special pouch: the marsupium. The reason that marsupials have this pouch is because the babies emerge at a much early stage of development than other mammals. The gestation period (the length of the time the offspring spend developing inside the mother) is relatively short for marsupials because unlike most mammals, marsupials do not share a blood supply with the mother to gain nutrients during gestation. Instead, they rely on a system similar to the yoke of an egg for nutrients for the developing young during this time. After birth, the young are still completely helpless and extremely small. They climb from the birth canal into their mother's pouch where the nipples are (marsupials are still mammals, so that means milk for the young) and continue to develop. Even when they have grown large enough to venture outside of the pouch, the young animals return to it to nurse and for safety in some situations.