This particular part of Brooks' technique is appealing. A point I touched on before with The Sword of Shannara, was the depth of the characters. The characters are full with histories that seem to never end. An example is Allanon. His past went way back to the time of Bremen.
Every important character has a past that seems to go on and on.
This trait helps the reader become more attached to the character as they read more. The more books you read, the more you understand a character. Terry Brooks is very good at making the character round. Not everyone enjoys Terry Brooks, but I couldn't find any specific criticism about his style in general. DR Terry Brooks' first book is exactly that, his first book.