>Having a property or method that is not in the parent class can be indicative of a poor class design. The purpose of the highest class level is to define the public interface for all of its subclasses. Introducing a new property or method down the inheritence tree causes the interface to change within the particular class.

I would say that was a little extreme, Jim. All that's necessary is that the child is-a parent. It's not necessary that a parent is-a child (with respect to interface).

There are two main reasons to subclass: specialization and extension. Both reasons allow adding private/protected properties and methods. The second reason allows adding public methods.

What you're describing is called "class flattening," and can itself be indicative of bad design.