Don't ask me why, but it seems that spotify is linked to libcups.so.2: /opt/spotify/spotify: error while loading shared libraries: libcups.so.2: cannot open shared object file: No such file or directory So I think that it should RDEPEND on net-print/cups. I did ldd -v /opt/spotify/spotify and revdep-rebuild to be sure that there is not another library which is linking to libcups.so.2. Emerging net-print/cups solved this.
fixed in spotify-0.8.4.103-r3