CM11: Nexus 10 boots forever
Boot into recovery and get a shell (e.g. adb shell on the PC connected via USB). Then format the userdata partition:

mke2fs -t ext4 /dev/block/platform/dw_mmc.0/by-name/userdata


Afterwards, simply reboot and it should work fine.

The reason why this is necessary is simply (thanks to @utkanos in #cyanogenmod for explaining it): When unlocking the bootloader using fastboot oem unlock it erases all data but doesn't create a proper filesystem.