There are a number of updates that need to be made for 4.1 to both the handbooks and the Codex.
There are several posts on Make/Core about changes in 4.1 that affect the developer handbooks:
Themes: New Template Tags in 4.1
Shared Terms: An Update on Shared Term Splitting
Customizer API Improvements: JS/Underscore-template-rendered Custom Customizer Controls in WordPress 4.1
I still need to add a list of Codex pages here that will need to be updated with the content from these posts.
The following are the pages that need updated or created in the Codex.
If you would like to help, please leave a comment indicating which page you would like to work on.
New default theme: