My situation is very similar to Shutdown Hangs for 5 Minutes on Kubuntu 14.04 but I am using 14.10 with a gnome environment instead of KDE (Ubuntu with gnome package added not the native version). That alone shouldn't be causing it as I have coworkers with same hardware and setup with no issues.
The verbose shutdown screen prints the same line
* Asking all remaining processes to terminate... * All Processes ended within 300 seconds... nm-dispatcher.action: Caught signal 15, shutting down... * Deactivating swap... mount: / is busy * Will now halt [ 486.185416] reboot: Power down
Unlike the original poster I don't have modem manager installed and there isn't anything outputted between nm-dispatcher and deactivating the swap for me to use as a guideline :/
If anyone knows some things to check for or a way to force a shutdown after 60 seconds of hanging vs 300 it would be much appreciated.