Description of problem: systemd should ship a file /etc/yum/protected.d/systemd with "systemd" in it (and/or any other package names which really shouldn't be removed). See "man yum.conf" etc. This is somewhat more important/useful/needed now because systemd has a much bigger set of requires than sysvinit ever did (and it's growing). So it's much more likely that someone could do "yum remove -r libmicrohttpd" or whatever.
Added into 195-4.
systemd-195-6.fc18 has been submitted as an update for Fedora 18. https://admin.fedoraproject.org/updates/systemd-195-6.fc18
Package systemd-195-6.fc18: * should fix your issue, * was pushed to the Fedora 18 testing repository, * should be available at your local mirror within two days. Update it with: # su -c 'yum update --enablerepo=updates-testing systemd-195-6.fc18' as soon as you are able to. Please go to the following url: https://admin.fedoraproject.org/updates/FEDORA-2012-17924/systemd-195-6.fc18 then log in and leave karma (feedback).
systemd-195-6.fc18 has been pushed to the Fedora 18 stable repository. If problems still persist, please make note of it in this bug report.