Most of the aspects are negative, but some are positive. Positive aspects would be the practice of Shamanism, paganism, healing, magick and religious beliefs, which we keep hidden to avoid criticism. In some family circles showing compassion, empathy or any emotions would be considered a weakness. Or perhaps you have hidden many of those qualities because of traumatic events in your life. You would feel vulnerable if you allowed yourself to display these emotions, so you have built a wall around your "true" self.
However, the shadow self is mainly composed of all of your suppressed negative emotions, desires and feelings which we experience. It holds outdated social ideas, gender based, sexual, racial stereotypes and prejudices. It feeds on discouragement, depression, fears and doubts.
The shadow self begins building the day we are born, it will exist until the day we die. It grows each time we dwell on negative feelings. It is our opposite and becomes an opposition. It will gain power when we deny its existence, and will attempt to dominate to control our lives. The shadow self wants you to fail in every aspect of your life, including the practice of shamanism.
These aspects of your personality will rise to the front of your consciousness as different beings or inanimate objects which represent a danger to you while you are on your journeys. You must learn to confront those fears and emotions. Each person will need to handle these aspects differently.
In order to be balanced a shaman must have both positive and negative aspects, which is where the shadow self plays an important role. A good example which requires the negative side would be to keep you from being overly compassionate which will allow you to be deceived by every person who tells you a sob story.
You can find more information on the Shadow Self in my book, Celtic shamanism: A Contemporary Guide for the Beginner.