There is an "Also share on Google+" option when you publish a comment under a YouTube video and proposals to make sure to untick the "Also share on Google+" option, which is currently on by default if you don't want your comment to be shared to Google+.
I was sure that not sharing just means that the comment will not be tracked in my Google+ journal but it will be visible in YouTube. However, I have noticed now that my comments are ghosted. They are not visible outside my account. I see the same "Also share on Google+" option when I make a comment in the YouTube blog.
"You control whether your comment will appear only here or also on Google+. Either way, it's visible only to the circles and people you choose"
pops up when I make a comment there. So, it seems that both Google applications ghost your comments, only YouTube does not advertise this fact.
Who can see my comments on YouTube if I do not G+ share them?
Can I retrieve the list of all my privately posted comments (I posted with option unticked)?
Wait, I cannot even see the "Also share on Google+"-shared on YouTube. I can see them only in my Google+.
How do people publish the comments at all?
It seems that they are only visible if I reply to somebody. I see that this is discussed in YouTube Help Forum yet the majority remains unaware of that because, once you see your comment published, you relax. Nevertheless, you still can answer which difference "Also share on Google+" is supposed to make.