I recently ran into a REQUIRED_USE constraint that required I select between berkdb and gdbm for an email client.
Looking through our profiles, I see we have both berkdb and gdbm enabled "globally". default/linux/make.defaults:USE="berkdb crypt ipv6 ncurses nls pam readline ssl tcpd zlib" releases/make.defaults:USE="acl gdbm nptl unicode" Is there any reason to have these USE flags enabled globally? These USE seem pretty package-specific in scope. On my system, they are used by around a dozen of 1000+ installed packages. I think it might make sense to migrate them to appropriate IUSE defaults, or leave them disabled where they do not provide critical functionality.
