Since a raw ChangeLog won't really do as release notes for anything except a tiny minor release of Evergreen, here's a list of items to include when compiling notes for a major release.
Installation/upgrade requirements and changes
Major new versions of dependent packages such as Postgres
Anything that would count as a major gotcha when upgrading – sure, these would also be included in the upgrade documentation, but this is one case where repetition can help.
Anything that would count as a required configuration task after upgrading.
New modules
Major architectural changes
New functionality to existing modules
Acquisitions
Cataloging
Vandelay
Circulation
Receipt printing
Holds management
OPAC
Search subsystem
Reports
Serials
Staff client (general)
Security considerations and bugfixes
Significant bugfixes (in principle, a list of all bugs fixed for the first time in the new release could be generated, but that might be better off in an appendix so as to not overwhelm the main release notes with a lot of trivia. Of course, what counts as a significant bugfix? YMMV).
I18N
New/deleted translations
Administrative settings and details
New/changed/deleted administration and configuration pages
New/changed/deleted server-side configuration files and settings
New/changed/deleted library settings
New/changed/deleted user setting types
New/changed/deleted global and internal flags (if relevant to the average user or Evergreen administrator)
New/changed/deleted user permissions
New/changed/deleted cronjobs and residents of /openils/bin
Credits (Galen's suggestion - include the names of libraries that have sponsored work and the names of individuals who have contributed patches.)