What are good themes to start from when creating a custom WordPress theme?
So far I have found:
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I think it's useful to start with childthemes
You can "extend" the default theme (twentyeleven). While using child themes the learning curve is smaller and it's not required to use all the files. Sure you can create a child theme from the Toolbox theme too. If you're able to modify the header and footer you got already 50% from your design. Be sure you need to know CSS or you get stuck while WP theming. Maybe you can create a theme for a "Page" first.
Toolbox is a great theme released by Automattic, more specifically, Ian Stewart of ThemeShaper. It's a great start that keeps "the WordPress way" at its core. I would caution you not to stray from accepted and best practices by starting with a theme that doesn't hold true to good practices.
Toolbox is an authorized theme of the company closest to WordPress itself, and so it is my safest and best recommendation for starting out on your theme creation journey. :)
You might want to start out by creating a child theme of Toolbox.