What was unusual about Jonas and the new child?
If by the new child you mean Gabriel, then I would say that Gabriel is pale-eyed. Although Jonas is also pale-eyed, this is a rarity in the Community. There is a suggestion that the two of them may have shared the same birth-mother.
Jonas's most unusual feature is his unusual eyes, they are pale, while everyone else’s are brown, and he has the ability to see things change in a way that he cannot explain.