I recently used fedup to upgrade my Fedora 19 machine to Fedora 20. However, after upgrading to Fedora 20 (which includes GNOME 3.10), i noticed that my normal keyboard shortcut (CTRL + ALT + L) was not locking the screen like it has previously.

After bashing the alt, control and L keys on my keyboard for a few moment, i figured out that in GNOME 3.10, the default shortcut to lock the screen is now changed to SUPER + L.

Note, however, that it is trivial to change this back (or to whatever other key combination you want)using the Keyboard preferences in the GNOME Settings.