You can try this -

sudo apt-get remove abiword abiword-common abiword-plugin-grammar abiword-plugin-mathview bison blueman brltty-x11 catfish elementary-icon-theme exo-utils flex gigolo gimp gimp-data gmusicbrowser gnome-icon-theme-full gnome-system-tools gnome-time-admin gnumeric gnumeric-common gnumeric-doc gstreamer0.10-gnomevfs gthumb gthumb-data gtk2-engines-pixbuf gtk2-engines-xfce indicator-application-gtk2 indicator-messages-gtk2 indicator-sound-gtk2 indicator-status-provider-pidgin leafpad libabiword-2.8 libaiksaurus-1.2-0c2a libaiksaurus-1.2-data libaiksaurusgtk-1.2-0c2a libao-common libao4 libaudio-scrobbler-perl libbabl-0.0-0 libclutter-1.0-0 libclutter-1.0-common libclutter-gtk-1.0-0 libcogl-common libcogl5 libconfig-inifiles-perl libencode-locale-perl libept1 libexo-1-0 libexo-common libfile-listing-perl libfont-afm-perl libgarcon-1-0 libgarcon-common libgdome2-0 libgdome2-cpp-smart0c2a libgegl-0.0-0 libgimp2.0 libglade2-0 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 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-ssl-perl libjpeg-progs libkeybinder0 liblink-grammar4 libloudmouth1-0 liblwp-mediatypes-perl liblwp-protocol-https-perl libmad0 libmailtools-perl libnet-dbus-perl libnet-http-perl libnet-ssleay-perl liboobs-1-5 libopenexr6 libotr2 libots0 libpolkit-gtk-1-0 libsexy2 libtagc0 libthunarx-2-0 libtie-ixhash-perl libtimedate-perl libtumbler-1-0 liburi-perl libwv-1.2-3 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 link-grammar-dictionaries-en m4 mpg321 murrine-themes orage parole pastebinit pidgin pidgin-data pidgin-libnotify pidgin-microblog pidgin-otr plymouth-theme-xubuntu-logo plymouth-theme-xubuntu-text python-configobj python-glade2 quadrapassel ristretto screensaver-default-images synaptic system-tools-backends tango-icon-theme tango-icon-theme-common tcl8.5 thunar thunar-archive-plugin thunar-data thunar-media-tags-plugin thunar-volman ttf-droid ttf-lyx tumbler tumbler-common xchat xchat-common xfburn xfce-keyboard-shortcuts xfce4-appfinder xfce4-cpugraph-plugin xfce4-dict xfce4-fsguard-plugin xfce4-indicator-plugin xfce4-mailwatch-plugin xfce4-mixer xfce4-mount-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-smartbookmark-plugin xfce4-systemload-plugin xfce4-taskmanager xfce4-terminal xfce4-utils xfce4-verve-plugin xfce4-volumed xfce4-weather-plugin xfconf xfdesktop4 xfdesktop4-data xfwm4 xfwm4-themes xscreensaver xscreensaver-data xscreensaver-gl xubuntu-artwork xubuntu-default-settings xubuntu-desktop xubuntu-docs xubuntu-icon-theme xubuntu-wallpapers && sudo apt-get install ubuntu-desktop

Source: http://www.psychocats.net/ubuntu/puregnome

Update: on (X)Ubuntu 13.10 the code above might yield an error on packages libabiword-2.8 and ttf-droid, so you might actually have to run this:

sudo apt-get remove abiword abiword-common abiword-plugin-grammar abiword-plugin-mathview alacarte bison blueman brltty-x11 catfish espeak exo-utils flex fonts-droid fonts-lyx gigolo gmusicbrowser gnome-system-tools gnome-time-admin gstreamer0.10-gnomevfs gthumb gthumb-data gtk2-engines-pixbuf indicator-application-gtk2 indicator-sound-gtk2 leafpad libabiword-3.0 libbison-dev libdigest-crc-perl libexo-1-0 libexo-common libexo-helpers libfl-dev libgarcon-1-0 libgarcon-common libgdome2-0 libgdome2-cpp-smart0c2a libglade2-0 libgnomevfs2-0 libgnomevfs2-common libgnomevfs2-extra libgsf-1-114 libgsf-1-common libgstreamer-perl libgtk2-notify-perl libgtk2-trayicon-perl libgtkmathview0c2a libgtkspell0 libido-0.1-0 libindicate-gtk3 libintl-perl libjpeg-progs libjpeg-turbo-progs libkeybinder0 liblink-grammar4 libloudmouth1-0 libnet-dbus-perl liboobs-1-5 libotr2 libots0 librarian0 libsexy2 libtagc0 libthunarx-2-0 libtidy-0.99-0 libtie-ixhash-perl libtumbler-1-0 libunique-1.0-0 libvte-common libvte9 libwv-1.2-4 libxfce4ui-1-0 libxfce4ui-utils libxfce4util-bin libxfce4util-common libxfce4util6 libxfcegui4-4 libxfconf-0-2 libxml-parser-perl libxml-twig-perl libxml-xpath-perl lightdm-gtk-greeter link-grammar-dictionaries-en m4 orage parole pastebinit pavucontrol pidgin pidgin-data pidgin-libnotify pidgin-microblog pidgin-otr plymouth-theme-xubuntu-logo plymouth-theme-xubuntu-text python-configobj rarian-compat ristretto screensaver-default-images scrollkeeper shimmer-themes system-tools-backends tcl8.5 thunar thunar-archive-plugin thunar-data thunar-media-tags-plugin thunar-volman tumbler tumbler-common xbrlapi xchat xchat-common xfburn xfce-keyboard-shortcuts xfce4-appfinder xfce4-cpugraph-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-verve-plugin xfce4-volumed xfce4-weather-plugin xfce4-xkb-plugin xfconf xfdesktop4 xfdesktop4-data xfwm4 xscreensaver xscreensaver-data xscreensaver-gl xubuntu-artwork xubuntu-default-settings xubuntu-desktop xubuntu-docs xubuntu-icon-theme xubuntu-wallpapers && sudo apt-get install ubuntu-desktop

Which is the same as the first code, but with libabiword-3.0 instead of 2.8 and without ttf-droid.