I had this problem on my iMac Mid 2011 at work. I had updated to Yosemite from Mavericks. I did have the start-up issue where the load percentage capped out at 50%. I restarted several times and even left it up all night, but it never completely loaded. I decided to try the Safe mode and finally got the computer up and proceeded to run Permissions verifications and repairs. Afterward, I restarted again and have had no issues with the restart. This computer is on a company-wide server with approximately 20 Macs or which 5 or so have Yosemite and 2 of us have complaints still about the operation capacities of the OS. More questions regarding the server interaction. Hope I find answers on that soon.
