Fedora 21 Workstation added the ability to log in and run a Wayland session from the login screen (GDM), leaving the login screen itself running using the older X protocol. This is changing with a new feature in Fedora 22 enabling the login screen to run on Wayland by default.
For most users, there will be no visible difference between the Fedora 21 login screen running X, and the Fedora 22 login screen running Wayland. The goal in Fedora is to progress to the point of replacing X with Wayland wherever it appears, and the login screen is a vital first step towards this goal. The reasoning behind this is that the login screen is sufficiently isolated and contained, making it an ideal first place to use Wayland by default.
Even though some video device drivers — i.e. the proprietary Nvidia drivers — do not support the Wayland stack yet, you will still have access to the log in screen. In these cases, the login screen will fall back to using the older X protocol version seamlessly. Futhermore, if the Wayland version of the login screen does encounter an issue and crashes, it will also fall back to the X protocol version seamlessly.