Copy and paste the complete line below (adapted from Xubuntu instructions here):

sudo apt-get remove gnome-desktop-data gnome-system-tools gnome-time-admin  gthumb gthumb-data gtk2-engines-pixbuf indicator-application-gtk2 indicator-messages-gtk2 indicator-sound-gtk2 libconfig-inifiles-perl libdigest-crc-perl libencode-locale-perl libept1.4.12 libexo-1-0 libexo-common libexo-helpers libfile-listing-perl libfl-dev libfont-afm-perl libgarcon-1-0 libgarcon-common libgdome2-0 libgdome2-cpp-smart0c2a libgegl-0.0-0 libglade2-0 libgnomevfs2-0 libgnomevfs2-common libgnomevfs2-extra libgoffice-0.8-8 libgoffice-0.8-8-common libgsf-1-114 libgsf-1-common libgstreamer-perl libgtk2-notify-perl libgtk2-trayicon-perl libgtkmathview0c2a libgtkspell0 libhtml-form-perl libhtml-format-perl libhtml-parser-perl libhtml-tagset-perl libhtml-tree-perl libhttp-cookies-perl libhttp-daemon-perl libhttp-date-perl libhttp-message-perl libhttp-negotiate-perl libid3tag0 libido-0.1-0 libilmbase6 libio-socket-inet6-perl libio-socket-ssl-perl libjavascriptcoregtk-1.0-0 libjpeg-progs libjpeg-turbo-progs libkeybinder0 liblaunchpad-integration1 liblink-grammar4 libloudmouth1-0 liblwp-mediatypes-perl liblwp-protocol-https-perl libmailtools-perl libnet-dbus-perl libnet-http-perl libnet-ssleay-perl liboobs-1-5 libopenexr6 libotr2 libots0 librarian0 libsexy2 libsocket6-perl libtagc0 libthunarx-2-0 libtidy-0.99-0 libtie-ixhash-perl libtimedate-perl libunique-1.0-0 liburi-perl libvte-common libvte9 libwebkitgtk-1.0-0 libwebkitgtk-1.0-common libwv-1.2-4 libwww-perl libwww-robotrules-perl libxfce4ui-1-0 libxfce4util-bin libxfce4util-common libxfce4util4 libxfcegui4-4 libxfconf-0-2 libxml-parser-perl libxml-twig-perl libxml-xpath-perl libxss1 lightdm-gtk-greeter lp-solve m4 plymouth-theme-ubuntustudio python-configobj python-glade2 python-gmenu rarian-compat screensaver-default-images sgml-data shimmer-themes thunar thunar-archive-plugin thunar-data thunar-media-tags-plugin thunar-volman ttf-droid xfburn xfce-keyboard-shortcuts xfce4-appfinder xfce4-cpugraph-plugin xfce4-datetime-plugin xfce4-dict xfce4-indicator-plugin xfce4-mailwatch-plugin xfce4-netload-plugin xfce4-notes xfce4-notes-plugin xfce4-notifyd xfce4-panel xfce4-places-plugin xfce4-power-manager xfce4-power-manager-data xfce4-quicklauncher-plugin xfce4-screenshooter xfce4-session xfce4-settings xfce4-systemload-plugin xfce4-taskmanager xfce4-terminal xfce4-utils xfce4-verve-plugin xfce4-volumed xfce4-weather-plugin xfce4-xkb-plugin xfconf xfdesktop4 xfdesktop4-data xfwm4 xscreensaver xscreensaver-data xscreensaver-gl xubuntu-icon-theme ubuntustudio* && sudo apt-get install ubuntu-desktop && sudo /usr/lib/lightdm/lightdm-set-defaults -g unity-greeter