For clarification, it's not possible to do exactly what was mentioned in the first post. Some node types are still distinctly different and can't be stacked in this way: Index, Area, Group, Book, Category, Attachment, Page.
However...
Each node type can have 1 Page as content. This page can take on the following page types: none, Template, Island, or Header.
Additionally,
Books, Categories, Attachments, and Pages can be added to Areas, Groups, Books, or Categories as children.
All these node types can receive their own comments without having to create Page-content first.