OK, eu consegui desinstalar o Ubuntu-desktop e aqui está o meu comando para fazê-lo. Use-o com sabedoria, pois ele também removerá o Brasero e o Compiz, bem como todos os pacotes dependentes.
sudo apt-get purge account-plugin-aim account-plugin-facebook account-plugin-flickr account-plugin-google account-plugin-jabber account-plugin-salut account-plugin-windows-live account-plugin-yahoo apport apport-gtk brasero brasero-cdrkit checkbox-gui compiz compiz-gnome compiz-plugins-default deja-dup deja-dup-backend-gvfs empathy example-content fonts-khmeros-core geoclue-2.0 gir1.2-unity-5.0 gnome-control-center-shared-data gnome-screensaver gnome-user-guide hud indicator-keyboard indicator-printers libaccount-plugin-1.0-0 libaccount-plugin-generic-oauth libaccount-plugin-google libbrasero-media3-1 libhud2 libufe-xidgetter0 libunity-action-qt1 libunity-core-6.0-9 libunity-misc4 libunity-protocol-private0 libunity-scopes-json-def-desktop libunity-webapps0 libunity9 libunityvoice1 light-themes mcp-account-manager-uoa nautilus nautilus-sendto nautilus-sendto-empathy nautilus-share oneconf plymouth-theme-ubuntu-logo qtdeclarative5-ubuntu-ui-extras-browser-plugin qtdeclarative5-ubuntu-ui-toolkit-plugin qtdeclarative5-unity-action-plugin shotwell software-center telepathy-indicator thunderbird-gnome-support ubuntu-artwork ubuntu-docs ubuntu-mono ubuntu-session ubuntu-settings ubuntu-sounds ubuntu-sso-client ubuntu-sso-client-qt ubuntu-wallpapers ubuntu-wallpapers-trusty ubuntuone-client-data unity unity-asset-pool unity-control-center unity-control-center-signon unity-greeter unity-lens-applications unity-lens-files unity-lens-friends unity-lens-music unity-lens-photos unity-lens-video unity-scope-audacious unity-scope-calculator unity-scope-chromiumbookmarks unity-scope-clementine unity-scope-colourlovers unity-scope-devhelp unity-scope-firefoxbookmarks unity-scope-gdrive unity-scope-gmusicbrowser unity-scope-gourmet unity-scope-guayadeque unity-scope-home unity-scope-manpages unity-scope-musicstores unity-scope-musique unity-scope-openclipart unity-scope-texdoc unity-scope-tomboy unity-scope-video-remote unity-scope-virtualbox unity-scope-yelp unity-scope-zotero unity-scopes-master-default unity-scopes-runner unity-services unity-settings-daemon unity-voice-service unity-webapps-common unity-webapps-qml unity-webapps-service usb-creator-gtk webaccounts-extension-common webapp-container webbrowser-app xul-ext-unity xul-ext-webaccounts xul-ext-websites-integration yelp account-plugin-aim account-plugin-facebook account-plugin-flickr account-plugin-google account-plugin-jabber account-plugin-salut account-plugin-windows-live account-plugin-yahoo apport apport-gtk brasero brasero-cdrkit checkbox-gui compiz compiz-gnome compiz-plugins-default deja-dup deja-dup-backend-gvfs empathy example-content fonts-khmeros-core geoclue-2.0 gir1.2-unity-5.0 gnome-control-center-shared-data gnome-screensaver gnome-user-guide hud indicator-keyboard indicator-printers libaccount-plugin-1.0-0 libaccount-plugin-generic-oauth libaccount-plugin-google libbrasero-media3-1 libhud2 libufe-xidgetter0 libunity-action-qt1 libunity-core-6.0-9 libunity-misc4 libunity-protocol-private0 libunity-scopes-json-def-desktop libunity-webapps0 libunity9 libunityvoice1 light-themes mcp-account-manager-uoa nautilus nautilus-sendto nautilus-sendto-empathy nautilus-share oneconf plymouth-theme-ubuntu-logo qtdeclarative5-ubuntu-ui-extras-browser-plugin qtdeclarative5-ubuntu-ui-toolkit-plugin qtdeclarative5-unity-action-plugin shotwell software-center telepathy-indicator thunderbird-gnome-support ubuntu-artwork ubuntu-desktop ubuntu-docs ubuntu-mono ubuntu-session ubuntu-settings ubuntu-sounds ubuntu-sso-client ubuntu-sso-client-qt ubuntu-wallpapers ubuntu-wallpapers-trusty ubuntuone-client-data unity unity-asset-pool unity-control-center unity-control-center-signon unity-greeter unity-lens-applications unity-lens-files unity-lens-friends unity-lens-music unity-lens-photos unity-lens-video unity-scope-audacious unity-scope-calculator unity-scope-chromiumbookmarks unity-scope-clementine unity-scope-colourlovers unity-scope-devhelp unity-scope-firefoxbookmarks unity-scope-gdrive unity-scope-gmusicbrowser unity-scope-gourmet unity-scope-guayadeque unity-scope-home unity-scope-manpages unity-scope-musicstores unity-scope-musique unity-scope-openclipart unity-scope-texdoc unity-scope-tomboy unity-scope-video-remote unity-scope-virtualbox unity-scope-yelp unity-scope-zotero unity-scopes-master-default unity-scopes-runner unity-services unity-settings-daemon unity-voice-service unity-webapps-common unity-webapps-qml unity-webapps-service usb-creator-gtk webaccounts-extension-common webapp-container webbrowser-app xul-ext-unity xul-ext-webaccounts xul-ext-websites-integration yelp