About TWO months apart downloading Yosemite, BOTH my iMac and my MacBook Pro wouldn't recognize my iCloud password on rebooting. I tried several times with both, then selected skip (thank goodness you CAN skip!). Then after it fully rebooted I went to preferences then iCloud. It was STILL loading slowly so I logged into iCloud.com, and signed-in there. THEN I went BACK to preferences and iCloud loaded faster this time and I was able to enter my password. Whew! With two different devices two months apart (didn't download to laptop right away) this seems to be a programming glich.
MacBook
Pro, OS X Yosemite (10.10.2)