I can automatically post my YouTube likes/comments to my Facebook & Twitter accounts but cannot find the same thing for Google Plus. It's funny! Below is the settings page for sharing of a YouTube account.
If you know how to do it, please share.
When you're logged in to your Google account (and you have G+ one) the "thumbs up" icon beneath the video is replaced with "G+ Share". Clicking that will cause the video clip to appear in your G+ stream.