It seems to be an issue. For me it's a major f***ing PITA, I have a number of videos posted there on my own YT channel (including one which has had over 300,000 views) and now I am forbidden from replying to any comments.
The reply function seems to be available *sometimes* but it is not available to account holders, apparently.
Thanks for that Daffy, I have only just discovered this kerfuffle and am trying to work it out. It looks pretty complicated but seems to center around Google's never ending quest to tap into peoples personal info.
Some of you such as Daffy seem to be misunderstanding.
It's the button to reply to another comment that is now missing.
Just noticed this myself though as I do have a Google+ link I am provided with the comment box. Quite idiotic and frustrating. Commenting is all I am able to do on youtube beside enjoy the videos as I lack the know how and program to make them.
Hmm interesting development. I just commented on a video and now I see a reply link at the bottom of the first two posts including my own, the two most recent. Perhaps it's related to the age of posts?