Looks like they're forcing Google Plus on everyone with the new comment system, along with a lot of other YouTube features that you'll have to go through Plus to view/use. Seems they're making a last ditch effort with this integration. Probably because Google Plus has been such an utter failure since its launch.
Let me just say, this is probably one of the worst changes YouTube's done so far. All those awful front page and channel design changes don't compare with this goof. smh