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