The first Fedora.Next release, Fedora 21, is out. Upgrade advice follows.
In a word: backup everything, especially configurations under /etc and each user’s $HOME. Pay particular attention to any custom .desktop files you have for 3rd party apps installed to /opt or /usr/local. I found that most of my configs got overwritten or cast aside due to a combination of rpm replacement magic and config schema changes.
Also, temporarily hide any third party yum repo files (e.g. you could move them to someplace yum won’t read like /etc/yum.repos.d/archive). This will prevent the upgrade failing due to the lack of support for Fedora 21 in those repos. You should check to make sure your favorite apps from any 3rd party repos are actually available in Fedora 21.
To upgrade a Fedora 20 system to Fedora 21 (as opposed to doing a fresh install) follow the instructions on the FedUp page on the Fedora Project Wiki. It’s no FreeBSD Handbook, but it’s the best we’ve got for now.
Be sure to follow the advice there for cleaning up after the upgrade. It will make things run much more smoothly going forward.
If you’re running the Gnome Shell as your desktop you’ll need to re-install most extensions, and find that a few aren’t available for Fedora 21 (like Gnome Shell Open Terminal, which allowed you to right-click on the desktop for a terminal).