Not long ago emerge --depclean wanted to remove less utility. gentoo.ru said, that it was removed from @system set and user should select proper pager (I prefer less) manually. To my mind such actions (changing, especially removing, package list in @system set) should begin from warning news message.
sys-apps/less has been removed from the base profile long time ago (in 2005): <http://sources.gentoo.org/cgi-bin/viewvc.cgi/gentoo-x86/profiles/base/packages?r1=1.33&r2=1.34> The effect that you see is caused either: - by a change in Portage's dependency resolution code, or - because less was removed from some dependency. *** This bug has been marked as a duplicate of bug 398295 ***