Release Notes: http://docs.fedoraproject.org/en-US/Fedora/21/html/Release_Notes/
Download: https://getfedora.org/en/workstation/download/
Some changes/improvements I found interesting:
GNOME 3.14
Update GNOME to the latest upstream release, 3.14
GCC49
Switch GCC in Fedora 21 to 4.9.x, rebuild all packages with it.
Headless Java
Server installations of Fedora should usually not pull in packages related to X system or sound subsystem. For this reason part of OpenJDK package has been split into headless subpackage which has smaller dependency chain. Fedora packages should be migrated to require java-headless instead of full java package when appropriate
Java 8
Make Java 8 (provided by OpenJDK 8 which is java-1.8.0-openjdk) the default Java runtime. The current default Java runtime (Java 7, provided by OpenJDK 7, java-1.7.0-openjdk) will be obsoleted and removed.