YouTube's comment system is getting a big overhaul from today, integrating with Google+ to create conversations and better moderation.
The big overhaul is that you'll now see comments sorted by people you know. So, if one of your Google+ friends comments on a video, that will be pushed up to the top. Likewise, comments from the video's creator, popular personalities, and better conversations will be moved up to the top as well.
If you're a video's creator, you can also now moderate comments to get rid of the cruft that YouTube is so well known for. Finally, you can also make comments private so only you and your Google+ friends can see them. The new comments are rolling out to YouTube over the coming days; as with most things Google, you may have to wait a little
Turning comments into conversations that matter to you [YouTube Official Blog]