Year ago I had a quite long discussions with a support team about the above topic. We created a theme that used more less the same html code base, but we made significant changes to design, homepage boxes, added more sidebar options, and so on. Even though the fact that HTML code is the same makes no difference to the end user, our theme was permanently rejected and disabled.
I just came across to some recent themes today that have exactly the same html, but different look and feel.
Has the review process changed since November 2010? If yes, that would be great.
I really don’t think that some authors that got 20+ themes are coding them from scratch any time they create a new one. It would just be a huge amount of work to use different classes and ids. Since every coder has their own “handwriting” simularities will always be there.
All the best