De acordo com a sugestão em este site, você pode obter o xubuntu-desktop puro:
Do Ubuntu Quantal:
sudo apt-get remove adium-theme-ubuntu apg appmenu-gtk appmenu-gtk3 appmenu-qt at-spi2-core bamfdaemon banshee banshee-extension-soundmenu banshee-extension-ubuntuonemusicstore baobab binfmt-support bluez-gstreamer branding-ubuntu brasero brasero-cdrkit brasero-common checkbox checkbox-gtk cli-common compiz compiz-core compiz-gnome compiz-plugins-default compiz-plugins-main-default compizconfig-backend-gconf deja-dup duplicity dvd+rw-tools empathy empathy-common eog evolution-data-server evolution-data-server-common example-content gbrainy gedit gedit-common geoclue geoclue-ubuntu-geoip ginn gir1.2-atspi-2.0 gir1.2-gnomebluetooth-1.0 gir1.2-gtksource-3.0 gir1.2-indicate-0.6 gir1.2-peas-1.0 gir1.2-totem-1.0 gir1.2-totem-plparser-1.0 gir1.2-wnck-3.0 gnome-bluetooth gnome-control-center gnome-control-center-data gnome-desktop3-data gnome-disk-utility gnome-font-viewer gnome-icon-theme-symbolic gnome-media gnome-nettool gnome-online-accounts gnome-orca gnome-power-manager gnome-screensaver gnome-screenshot gnome-search-tool gnome-session gnome-session-bin gnome-session-canberra gnome-session-common gnome-settings-daemon gnome-system-log gnome-system-monitor gnome-terminal gnome-terminal-data gnome-user-share gnome-utils-common growisofs gstreamer0.10-gconf gvfs-backends gwibber gwibber-service gwibber-service-facebook gwibber-service-identica gwibber-service-twitter hwdata ibus-gtk3 indicator-appmenu indicator-datetime indicator-power indicator-session intel-gpu-tools libappindicator0.1-cil libarchive1 libatk-adaptor libatspi2.0-0 libaudio2 libbamf0 libbamf3-0 libboost-serialization1.46.1 libbrasero-media3-1 libcamel-1.2-29 libcanberra-pulse libcdio-cdda0 libcdio-paranoia0 libcdio10 libcompizconfig0 libdbus-glib1.0-cil libdbus1.0-cil libdbusmenu-qt2 libdconf-dbus-1-0 libdconf-qt0 libdconf0 libdecoration0 libebackend-1.2-1 libebook1.2-12 libecal1.2-10 libedata-book-1.2-11 libedata-cal-1.2-13 libedataserver1.2-15 libedataserverui-3.0-1 libexempi3 libfolks-telepathy25 libfolks25 libgail-3-common libgail-common libgconf2.0-cil libgdata-common libgdata1.7-cil libgdata13 libgdiplus libgdu-gtk0 libgeoclue0 libgexiv2-0 libgif4 libgkeyfile1.0-cil libglew1.5 libglewmx1.5 libglib2.0-bin libglib2.0-cil libglib2.0-data libgmime-2.4-2 libgmime2.4-cil libgnome-control-center1 libgnome-desktop-3-2 libgnome-media-profiles-3.0-0 libgnome-menu2 libgnome2-common libgnomekbd-common libgnomekbd7 libgoa-1.0-0 libgpgme11 libgpod-common libgpod4 libgtk-sharp-beans-cil libgtk2.0-cil libgtkmm-3.0-1 libgtksourceview-3.0-0 libgtksourceview-3.0-common libgtkspell3-0 libgudev1.0-cil libgweather-3-0 libgweather-common libgwibber-gtk2 libgwibber2 libhyphen0 libidl0 liblaunchpad-integration1.0-cil liblircclient0 liblouis-data liblouis2 libmetacity-private0 libmhash2 libmission-control-plugins0 libmono-addins-gui0.2-cil libmono-addins0.2-cil libmono-cairo4.0-cil libmono-corlib4.0-cil libmono-csharp4.0-cil libmono-i18n-west4.0-cil libmono-i18n4.0-cil libmono-posix4.0-cil libmono-security4.0-cil libmono-sharpzip4.84-cil libmono-system-configuration4.0-cil libmono-system-core4.0-cil libmono-system-drawing4.0-cil libmono-system-security4.0-cil libmono-system-xml4.0-cil libmono-system4.0-cil libmono-zeroconf1.0-cil libmtp-common libmtp-runtime libmtp9 libmysqlclient16 libmythes-1.2-0 libneon27-gnutls libnotify0.4-cil libnux-1.0-0 libnux-1.0-common liboauth0 liborbit2 liboverlay-scrollbar-0.2-0 liboverlay-scrollbar3-0.2-0 libpeas-1.0-0 libpeas-common libprotobuf7 libprotoc7 libpth20 libqt4-dbus libqt4-declarative libqt4-network libqt4-opengl libqt4-script libqt4-sql libqt4-sql-mysql libqt4-svg libqt4-xml libqt4-xmlpatterns libqtbamf1 libqtcore4 libqtdee2 libqtgconf1 libqtgui4 libquvi0 libraptor2-0 librasqal3 librdf0 libreoffice-base-core libreoffice-calc libreoffice-common libreoffice-core libreoffice-draw libreoffice-emailmerge libreoffice-gnome libreoffice-gtk libreoffice-help-en-us libreoffice-impress libreoffice-math libreoffice-style-human libreoffice-writer librest-0.7-0 librsync1 libsdl1.2debian libsdl1.2debian-pulseaudio libstlport4.6ldbl libsyncdaemon-1.0-1 libtaglib2.0-cil libtelepathy-farsight0 libtelepathy-logger2 libtextcat-data libtextcat0 libtotem-plparser17 libtotem0 libubuntuone-1.0-1 libubuntuone1.0-cil libunique-3.0-0 libunity-2d-private0 libunity-core-4.0-4 libunity-misc4 libwmf0.2-7-gtk libwnck-3-0 libwnck-3-common libyajl1 libzeitgeist-1.0-1 light-themes media-player-info metacity metacity-common mono-4.0-gac mono-gac mono-runtime mousetweaks mysql-common nautilus nautilus-sendto nautilus-sendto-empathy nautilus-share notify-osd notify-osd-icons nux-tools obexd-client overlay-scrollbar plymouth-theme-ubuntu-logo protobuf-compiler pulseaudio-module-bluetooth pulseaudio-module-gconf python-brlapi python-configglue python-dateutil python-egenix-mxdatetime python-egenix-mxtools python-farsight python-indicate python-libproxy python-louis python-papyon python-protobuf python-pyatspi2 python-pyinotify python-speechd python-support python-telepathy python-twisted-names python-ubuntuone-client python-ubuntuone-control-panel python-ubuntuone-storageprotocol python-uno python-wnck qdbus qt-at-spi seahorse shotwell sni-qt ssh-askpass-gnome telepathy-butterfly telepathy-gabble telepathy-haze telepathy-idle telepathy-indicator telepathy-logger telepathy-mission-control-5 telepathy-salut thunderbird-gnome-support tomboy totem totem-common totem-mozilla totem-plugins ubuntu-artwork ubuntu-desktop ubuntu-docs ubuntu-mono ubuntu-sounds ubuntu-system-service ubuntu-wallpapers ubuntuone-client ubuntuone-client-gnome ubuntuone-control-panel ubuntuone-control-panel-gtk ubuntuone-couch ubuntuone-installer unity unity-2d unity-2d-launcher unity-2d-panel unity-2d-places unity-2d-spread unity-asset-pool unity-common unity-lens-applications unity-lens-files unity-lens-music unity-scope-musicstores unity-services uno-libs3 ure vino whois wodim xdiagnose xfonts-mathml zeitgeist zeitgeist-datahub zeitgeist-extension-fts
Do Kubuntu Quantal:
sudo apt-get remove akonadi-backend-mysql akonadi-server akregator amarok amarok-common amarok-utils appmenu-gtk appmenu-gtk3 appmenu-qt apport-kde apturl-kde ark bluedevil cdparanoia cdrdao docbook-xsl dolphin dragonplayer dvd+rw-tools freespacenotifier gnupg-agent gnupg2 gpgsm growisofs gstreamer0.10-qapt gtk2-engines-oxygen gwenview ibus-qt4 icoutils jockey-kde k3b k3b-data kaccessible kaddressbook kamera kate kate-data katepart kcalc kde-baseapps-bin kde-baseapps-data kde-config-gtk kde-config-touchpad kde-runtime kde-runtime-data kde-wallpapers-default kde-window-manager kde-workspace kde-workspace-bin kde-workspace-data kde-workspace-kgreet-plugins kde-zeroconf kdebase-runtime kdegames-card-data kdegraphics-strigi-analyzer kdelibs-bin kdelibs5-data kdelibs5-plugins kdemultimedia-kio-plugins kdenetwork-filesharing kdepasswd kdepim-groupware kdepim-kresources kdepim-runtime kdepim-strigi-plugins kdepim-wizards kdepimlibs-kio-plugins kdesudo kdm kdoctools kfind khelpcenter4 kinfocenter klipper kmag kmail kmix kmousetool knotes konsole kontact kopete kopete-message-indicator korganizer kpat kppp ksnapshot ksysguard ksysguardd ksystemlog ktimetracker ktorrent ktorrent-data kubuntu-debug-installer kubuntu-default-settings kubuntu-desktop kubuntu-docs kubuntu-firefox-installer kubuntu-netbook-default-settings kubuntu-notification-helper kubuntu-web-shortcuts kvkbd kwalletmanager language-selector-kde libakonadi-calendar4 libakonadi-contact4 libakonadi-kabc4 libakonadi-kcal4 libakonadi-kde4 libakonadi-kmime4 libakonadiprotocolinternals1 libarchive1 libassuan0 libattica0 libaudio2 libbluedevil1 libboost-program-options1.46.1 libcalendarsupport4 libcanberra-pulse libcln6 libclucene0ldbl libdbusmenu-qt2 libdebconf-kde0 libdiscid0 libdlrestrictions1 libdmtx0a libepub0 libeventviews4 libflac++6 libgadu3 libgif4 libgpgme++2 libgpgme11 libgpod4-nogtk libgps19 libgrantlee-core0 libhyphen0 libibus-qt1 libincidenceeditorsng4 libindicate-qt1 libiodbc2 libk3b6 libkabc4 libkateinterfaces4 libkatepartinterfaces4 libkblog4 libkcal4 libkcalcore4 libkcalutils4 libkcddb4 libkcmutils4 libkde3support4 libkdecorations4 libkdecore5 libkdegames5a libkdepim4 libkdepimdbusinterfaces4 libkdesu5 libkdeui5 libkdewebkit5 libkdgantt2 libkdnssd4 libkemoticons4 libkephal4abi1 libkexiv2-10 libkexiv2-data libkfile4 libkholidays4 libkhtml5 libkidletime4 libkimap4 libkio5 libkipi-data libkipi8 libkjsapi4 libkjsembed4 libkldap4 libkleo4 libkmanagesieve4 libkmbox4 libkmediaplayer4 libkmime4 libknewstuff2-4 libknewstuff3-4 libknotifyconfig4 libkntlm4 libkonq-common libkonq5-templates libkonq5abi1 libkontactinterface4 libkopete4 libkparts4 libkpgp4 libkpimidentities4 libkpimtextedit4 libkpimutils4 libkprintutils4 libkpty4 libkresources4 libkrosscore4 libksba8 libkscreensaver5 libksgrd4 libksieve4 libksieveui4 libksignalplotter4 libktexteditor4 libktnef4 libktorrent-l10n libktorrent3 libkunitconversion4 libkwineffects1abi2 libkworkspace4 libkxmlrpcclient4 liblastfm0 libmailcommon4 libmailtransport4 libmessagecomposer4 libmessagecore4 libmessagelist4 libmessageviewer4 libmhash2 libmicroblog4 libmpcdec6 libmsn0.3 libmtp-common libmtp-runtime libmtp9 libmuonprivate1 libmusicbrainz3-6 libmysqlclient16 libmythes-1.2-0 libneon27-gnutls libnepomuk4 libnepomukquery4a libnepomukutils4 libntrack-qt4-1 libntrack0 libokularcore1 libphonon4 libplasma-geolocation-interface4 libplasma3 libplasmaclock4abi2 libplasmagenericshell4 libpolkit-qt-1-1 libpoppler-qt4-3 libprison0 libprocesscore4abi1 libprocessui4a libpth20 libqalculate5 libqapt-runtime libqapt1 libqca2 libqca2-plugin-ossl libqgpgme1 libqimageblitz4 libqjson0 libqrencode3 libqt4-dbus libqt4-declarative libqt4-designer libqt4-help libqt4-network libqt4-opengl libqt4-qt3support libqt4-script libqt4-scripttools libqt4-sql libqt4-sql-mysql libqt4-sql-sqlite libqt4-svg libqt4-test libqt4-xml libqt4-xmlpatterns libqtassistantclient4 libqtcore4 libqtglib-2.0-0 libqtgstreamer-0.10-0 libqtgui4 libqtscript4-core libqtscript4-gui libqtscript4-network libqtscript4-sql libqtscript4-uitools libqtscript4-xml libqtwebkit4 libraptor2-0 librasqal3 librdf0 libreoffice-base-core libreoffice-calc libreoffice-common libreoffice-core libreoffice-draw libreoffice-emailmerge libreoffice-impress libreoffice-kde libreoffice-math libreoffice-style-human libreoffice-style-oxygen libreoffice-writer libsolid4 libsolidcontrol4abi2 libsolidcontrolifaces4abi2 libsoprano4 libssh-4 libstlport4.6ldbl libstreamanalyzer0 libstreams0 libsyndication4 libtag-extras1 libtaskmanager4abi2 libtemplateparser4 libtextcat-data libtextcat0 libthreadweaver4 libvirtodbc0 libweather-ion6 libxml2-utils libyajl1 libzip1 muon muon-installer muon-notifier muon-updater mysql-client-core-5.1 mysql-common mysql-server-core-5.1 ntrack-module-libnl-0 obexd-client odbcinst odbcinst1debian2 okular okular-extra-backends oxygen-cursor-theme oxygen-icon-theme oxygen-icon-theme-complete partitionmanager phonon phonon-backend-gstreamer pinentry-gtk2 pinentry-qt4 plasma-dataengines-addons plasma-dataengines-workspace plasma-desktop plasma-netbook plasma-scriptengine-javascript plasma-scriptengine-python plasma-widget-facebook plasma-widget-folderview plasma-widget-kimpanel plasma-widget-kimpanel-backend-ibus plasma-widget-menubar plasma-widget-message-indicator plasma-widget-networkmanagement plasma-widgets-addons plasma-widgets-workspace plymouth-theme-kubuntu-logo plymouth-theme-kubuntu-text printer-applet pulseaudio-module-bluetooth python-kde4 python-pyudev python-qt4 python-qt4-dbus python-sip python-uno qapt-batch qapt-deb-installer qdbus quassel quassel-data rekonq shared-desktop-ontologies software-properties-kde soprano-daemon system-config-printer-kde systemsettings uno-libs3 update-manager-kde ure usb-creator-kde userconfig virtuoso-minimal virtuoso-opensource-6.1-bin virtuoso-opensource-6.1-common wodim xfonts-mathml
De Quantum de Lubuntu:
sudo apt-get remove ace-of-penguins audacious audacious-plugins chromium-browser chromium-browser-l10n chromium-codecs-ffmpeg esound-common galculator gdebi gdebi-core giblib1 gir1.2-gnomebluetooth-1.0 gnome-bluetooth gnome-disk-utility gnome-mplayer gnome-user-share gpicview guvcview gvfs-backends hardinfo libarchive1 libaudclient2 libaudcore1 libaudio2 libaudiofile0 libavcodec53 libavformat53 libavutil51 libbinio1ldbl libcddb2 libcdio-cdda0 libcdio-paranoia0 libcdio10 libcompfaceg1 libcue1 libdca0 libdirectfb-1.2-9 libdiscid0 libdvdnav4 libdvdread4 libenca0 libesd0 libfaad2 libfluidsynth1 libfm-data libfm-gtk-data libfm-gtk1 libfm1 libgdu-gtk0 libgif4 libgpgme11 libgpod-common libgpod4 libgringotts2 libgsm1 libimlib2 liblircclient0 libmcrypt4 libmcs1 libmenu-cache1 libmhash2 libmms0 libmowgli2 libmtp-common libmtp-runtime libmtp9 libmusicbrainz3-6 libneon27-gnutls libobrender27 libobt0 libonig2 libopenal1 libopts25 libpisock9 libpostproc52 libpth20 libresid-builder0c2a libschroedinger-1.0-0 libsdl1.2debian libsdl1.2debian-alsa libsidplay2 libsvga1 libswscale2 libtar0 libts-0.0-0 libuniconf4.6 libunique-3.0-0 libva1 libvdpau1 libvpx0 libwebcam0 libwvstreams4.6-base libwvstreams4.6-extras libxvidcore4 lm-sensors lubuntu-artwork lubuntu-core lubuntu-default-settings lubuntu-desktop lubuntu-icon-theme lxappearance lxappearance-obconf lxde-common lxde-core lxdm lxinput lxkeymap lxlauncher lxmenu-data lxpanel lxpanel-indicator-applet-plugin lxrandr lxsession lxsession-edit lxshortcut lxtask lxterminal mplayer mtpaint ntp obconf obexd-client openbox openbox-themes osmo pcmanfm plymouth-theme-lubuntu-logo plymouth-theme-lubuntu-text python-support scrot sylpheed sylpheed-doc sylpheed-i18n sylpheed-plugins transmission tsconf uvcdynctrl uvcdynctrl-data wvdial xfonts-100dpi xpad && sudo apt-get install xubuntu-desktop
Sugere fazer sudo apt-get install xubuntu-desktop
, o que você já fez. Como converter o Ubuntu para o Xubuntu fala sobre as mudanças que você obter assim você deve editar o comando e ver o que você pode precisar.