Update : Finally it seems that the GPU is dead. I installed Debian testing with XFCE UI which is very smooth even n software rendering

The last week I was given an iMac 27" to repair, with Yosemite installed on it. It boots fine but was stuck at about 1/3 of the progress bar. Booting in safe mode works (left shift key pressed at startup), so it's not an hardware issue. After trying a lot of things (booting in verbose mode (command + V), reset SMC/PRAM, reinstalling system in recovery mode (command + R)), I found the solution thanks to this thread.

You have to boot in single user mode (command + S), then type :

# mount - uw /
# system_profiler SPExtensionsDataType > /tmp/a
# less a

Then search for "Not Signed" with "/" comamnd and goto next found iteration with "n" command.

We can see "EPSONUSBPrinter" and "hp_fax_io" are not signed, so delete these extensions and reboot like this :

# cd /System/Library/Extensions
# rm -rf name_of_the_kext.kext
# rm -rf /System/Library/Caches/*
# reboot

Be careful to not delete some other non signed core extensions (JMicronATA...). Select only external peripherals drivers. A blocking module was my first diagnostic, but impossible to know which is responsible until there is no really verbose boot mode (thanks Apple).

Update

There is one way to have access to logs during boot :

• Go in recovery mode
• Launch a terminal
• Type "resetpassword"
• Reset root password
• Reboot in safe mode
• Login as root (other)
• Enable SSH access
• Reboot
• Connect to your mac in SSH (ssh root@XXX)
• Read the log in /var/log/system.log