NM might mess up the pending-actions, which causes NM not to reach "startup complete" and nm-online to hang. This is the same issue as bug 1084554 in rhel-7, which itself was cloned from bug 1082045.
Fix pushed to upstream master as: http://cgit.freedesktop.org/NetworkManager/NetworkManager/commit/?id=fc1351504db557b942c121b480fca661162702f7
NetworkManager-0.9.9.0-34.git20131003.fc20 has been submitted as an update for Fedora 20. https://admin.fedoraproject.org/updates/NetworkManager-0.9.9.0-34.git20131003.fc20
Package NetworkManager-0.9.9.0-34.git20131003.fc20: * should fix your issue, * was pushed to the Fedora 20 testing repository, * should be available at your local mirror within two days. Update it with: # su -c 'yum update --enablerepo=updates-testing NetworkManager-0.9.9.0-34.git20131003.fc20' as soon as you are able to. Please go to the following url: https://admin.fedoraproject.org/updates/FEDORA-2014-4964/NetworkManager-0.9.9.0-34.git20131003.fc20 then log in and leave karma (feedback).
NetworkManager-0.9.9.0-35.git20131003.fc20 has been submitted as an update for Fedora 20. https://admin.fedoraproject.org/updates/NetworkManager-0.9.9.0-35.git20131003.fc20
NetworkManager-0.9.9.0-36.git20131003.fc20 has been submitted as an update for Fedora 20. https://admin.fedoraproject.org/updates/NetworkManager-0.9.9.0-36.git20131003.fc20
NetworkManager-0.9.9.0-37.git20131003.fc20 has been submitted as an update for Fedora 20. https://admin.fedoraproject.org/updates/NetworkManager-0.9.9.0-37.git20131003.fc20
NetworkManager-0.9.9.0-38.git20131003.fc20 has been submitted as an update for Fedora 20. https://admin.fedoraproject.org/updates/NetworkManager-0.9.9.0-38.git20131003.fc20
NetworkManager-0.9.9.0-38.git20131003.fc20 has been pushed to the Fedora 20 stable repository. If problems still persist, please make note of it in this bug report.