Canonical as replaced the default LightDM login manager on its Ubuntu platform with GDM login manager. The latest change is in line with the recent tweaks that are aiming to bring GNOME to Ubuntu.
Notably, this is not the first move by Canonical. The Ubuntu maker is, in fact, replacing most of the interface components of the open source platform with tools from GNOME. One such major transition was debuted back in June when the existing Unity 7 was replaced with GNOME desktop environment.
How to get back the original experience
In case if you are not comfortable with the new GDM login manager, you can bring back the eye-candy LightDM. You just need to pass the command
sudo dpkg-reconfigure gdm in the terminal emulator and then choose your preferred login manager.
Replacement of the default login manager is likely to bring some issues to your system as the LightDM might have some incompatibilities with the GNOME Shell. Nevertheless, you still have the chance to overcome the dull-looking login screen.