I believe in this context it, introspection is meant to provide some mechanism for determining what variables (and methods for accessing them) exist for a given object. For instance, if you have

vtkObject* obj;
cout << obj->GetClassName() << "\n";

You might see that you have a vtkPlane subclass of vtkObject, but without performing a downcast to the vtkPlane class, you have no way of knowing that the object has Normal and Origin members that are 3-tuples of doubles. This would provide a way for determining that information without downcasting, most likely by adding a few new methods to vtkObject.

Another implementation to consider is libsigc++ (http://libsigc.sourceforge.net/). Unlike Qt signals, it does not require a tool like moc to preprocess source code.