Hello Linux Geeksters. The initial plan was to enable Wayland by default on GNOME 3.12, the developers have decided to support Wayland as default only on GNOME 3.14, due to some issues, despite the fact that GNOME Shell 3.12 and Mutter already supports Samsung and Intel’s system compositor.
For now, GNOME 3.12 over Wayland lacks a bunch of important features and so, the developers will keep X.org as default, in order to provide a stability.
Most likely, the users will be able to start Wayland from the GDM, but it will not have all the features that GNOME 3.12 over X.org has. Also, they will keep working at it after the release of GNOME 3.12 and more features for Wayland will be made available, via the regular updates.
Also Canonical has rescheduled the implementation of Mir by default as a replacement for X.org, due to some performance issues.