While there's quite a good selection of theming docs on d.o, and covering some technical details of using a base/subtheme, there's not really a good section on the concepts and reasons behind this. I see lots of new users in #drupal-support trying to write their own theme from scratch, which can be an interesting intellectual exercise but not realistically what most people should be doing. Why reinvent the wheel!

Deliverable:
Create a handbook page in the theming section about using a base theme. This should cover:
- what is a base theme (vs. starter theme?)
- why would you want to use one?
- what features do they tend to provide?
- are there any times you shouldn't use one?
- overview of the popular base themes and things to consider when choosing one (note: not a full comparison of base themes, that would be a whole other larger task)

Resources:
http://drupal.org/node/805968
http://drupal.org/node/225125
http://www.appnovation.com/drupal-base-themes
http://mogdesign.eu/blog/19-base-themes-for-drupal/