The rightmost toolbar group of Rhino 6 is named ‘New in V6’. It lists roughly half of the new commands.
All new commands are listed in this file: https://docs.mcneel.com/rhino/6/help/en-us/commandlist/newinrhino.htm
The file is worth reading because it describes many useful innovations. Links in this file take you to sometimes rough and unfinished descriptions of the new commands.
For example, if you type “RH-46564” in the search window of the YouTrack website, it displays description of the bug called “RH-46564”. “RH” means Rhino and “46564” is the bug number.
If you type “GetIssueState RH-46564” in Rhino 6.4 command prompt window, Rhino returns “False”, which means that the bug has not been fixed.
According to YouTrack, Rhino had 12539 known bugs on 2018-06-07. Rhino users do not know how many of these bugs have been fixed. YouTrack has nifty bug statistics in its top-right corner, but Rhino users do not have permission to view these statistics.