Whenever I try to shut down my iMac (3.2 GHz Intel Core i3, running 10.7.3), it closes out all programs, goes to the gray screen with the spinning ball and just hangs there. This occurs in restart or shut down mode. Any thoughts?
If you have any attached external TB, FW or USB devices other than keyboard & mouse, dismount them first, (if mass-storage devices,) then shutdown & see if it completes.
I have a client (my Mom) whose Mac Mini had the same symptoms and found that she'd kept a USB thumb drive physically attached & had forgotten about it. It had been there for some months, but at some point something happened to the drive (out of space? corrupted directory?) and when it was removed for troubleshooting, the Mac shut down normally. She'd gotten into the habit of forcing the power-off, too. Ouch.
Try booting into Safe Mode and see if the shutdown issue exist.
You can also open Activity Monitor in the Utilities folder. Select All Processes from the Processes dropdown menu. Click twice on the % CPU column header to display items in descending order. If you find a process using a large amount of CPU time, then select the process and click on the Quit icon in the toolbar. Click on the Force Quit button to kill the process.
Causes could be:
Hope that helps.