Auto-rotate only for YouTube app, disabled otherwise I find auto-rotate really annoying so I generally have it turned off.  

Unfortunately, when I'm playing a YouTube video, it's equally annoying when I try to tilt the screen and then remember I have it turned off  :) 

I would like a way to have auto-rotate working as usual when a YouTube video is playing, otherwise disabled.  

Is that possible? I would like it all handled automatically, I'm not really interested in apps or widgets where I have to toggle a setting manually.

P.S. My phone is a Galaxy Nexus.
Alternatively, my default recommendation for cases like this: Tasker. Use a profile like this (abstract):

Condition: App (select "Youtube", and optionally other apps you would like to have autorotate turned on)

Task: Display->Display Rotation: Set On

This way, whenever one of the selected apps runs in foreground, autorotate is turned on -- and as soon as that condition is no longer met, autorotate is turned to whatever it was set to before. To make sure to have it turned off afterwards, you can add a so called "exit task":

Exit: Display->Display Rotation: Set Off

Hm, looks like currently I answer every 2nd question with "Tasker"...