Comments from your circles are surfaced; better moderation tools for video producers
If you've spent any amount of time on Youtube you know the comments aren't always the highest of quality, and Google is hoping to mitigate that issue by transitioning Youtube comments to be powered by Google+. Starting this week with the channel discussion tab and coming to all videos later this year, all comments on Youtube will be transitioned to Google+ discussions.
Moving to Google+ comments not only pushes towards a "real name" policy on Youtube, but also gives Google the opportunity to better sort and feature the comments and discussions that are most relevant to you. Comments that you care about most — from people you know, follow or are engaged in discussion with — will be shown at the top of the list.
It also means that you can create discussions around a video that show up in the comments, but only for you and a limited set of your friends. You can restrict comment threads to a single person or a tight group of people in a circle or two if you don't want to publicly comment on a video. Tools for content producers will also be improved, with better ways to moderate comments before and after they are posted.
Source: Official Youtube Blog