YouTube received flak for requiring users to have Google+ accounts to comment on videos and when they moved the comment notifications to Google+ notification page. Google’s video streaming service seemed to have listened to its large user base and introduced at Comments Page to help video creators manage, view and respond to comments in one place.
At the moment, the users will be required to click on the video page and reply to the comments there. But the company intends to allow for inline replying and expanding all replies on the Comments Page soon.
Source: YouTube Creator Blog