MacBook Pro Boots only with a pressed key
I just found out it seems the power button is "stuck" and pressing option seems to prevent the laptop going into shutdown, as when I booted into El Capitan, a few seconds after I let the option key go, I saw the shutdown dialog for a couple of seconds... I'm going to take it to the repair warranty to recheck the keyboard topcase.

Update: The problem was with a faulty logic board power button circuit. In my case the whole board was replaced as it was still in the warranty period.