The important thing to know about the default theme is that much of the code that runs the front-end pages is coded into the template itself. Any changes you make to the theme pages means that when the theme is updated, every change needs to be moved over to updated file. When a GeN4 theme design deviates greatly from the default theme, it makes updating it more complicated.
To make GeN4 easier to update and to keep design costs down, my low-end priced themes make only minor changes to the default theme. The changes I do mare are well marked to assist in updating. While this is a cost-efficient way to have a unique design for your GeN4 site, it does limit design options. Take a look at this diagram of the structure of the default homepage template. I can move some text areas around and change their shape somewhat from a separate css file, however, the basic structure of the page will remain.
By comparing the image of the GoodKarmaClicks theme to the diagram you can see how I used the default theme as the structure and was able to design a unique and distinctive look.