I've been wanting a simple method for managing dependent tasks for  
some time now, and only now did it occur to me that I could just  
implement a much simpler method using your current blocking mechanism.

The attached file, confusingly named org-depends.el, implements the  
following scheme:

  1. Any TODO which has incomplete child TODOs is blocked.

  2. If a parent TODO has the ORDERED property, it's children must be
     completed in order.  Undone siblings block later siblings.

  3. Blocked items are greyed out in the agenda list.

John