*** WARNING ***  Depclean may break link level dependencies.  Thus, it is
*** WARNING ***  recommended to use a tool such as `revdep-rebuild` (from
*** WARNING ***  app-portage/gentoolkit) in order to detect such breakage.
*** WARNING ***  
*** WARNING ***  Also study the list of packages to be cleaned for any obvious
*** WARNING ***  mistakes. Packages that are part of the world set will always
*** WARNING ***  be kept.  They can be manually added to this set with
*** WARNING ***  `emerge --noreplace <atom>`.  Packages that are listed in
*** WARNING ***  package.provided (see portage(5)) will be removed by
*** WARNING ***  depclean, even if they are part of the world set.
*** WARNING ***  
*** WARNING ***  As a safety measure, depclean will not remove any packages
*** WARNING ***  unless *all* required dependencies have been resolved.  As a
*** WARNING ***  consequence, it is often necessary to run 
*** WARNING ***  `emerge --update --newuse --deep world` prior to depclean.

Calculating dependencies... done!

Dependencies could not be completely resolved due to
the following required packages not being installed:

>=media-plugins/gst-plugins-alsa-0.10 required by media-libs/gst-plugins-base-0.10.11
>=media-plugins/gst-plugins-oss-0.10 required by media-libs/gst-plugins-base-0.10.11
>=media-plugins/gst-plugins-x-0.10 required by media-libs/gst-plugins-base-0.10.11

Have you forgotten to run `emerge --update --newuse --deep world` prior to
depclean?  It may also be necessary to manually uninstalled packages that no
longer exist in the portage tree since it may not be possible to satisfy their
dependencies.