Not forgiving your brother from the heart. Sin is serious. For I tell you that their angel in heaven always see the face of my Father in heaven. Not clear what it means. Obviously something shared in Matthew's worldview by his original readers.
Weight is quite clear.
They've got God himself on their side, and God himself is their life and glory, so be careful whom you take on. To depreciate one of God's little ones is to depreciate Jesus, and is to put you in opposition to God's will. The meaning is made quite explicit for us in vs. 14. It is against God's will to lose a single one of them. That is what the parable of the lost sheep is about.