The recent change in the YouTube comment system, more specifically, the merge with Google+ has created a tremendous amount of backlash.
"Due to the recent changes of the YouTube comment system I'm forced to disable comments. Since front page/top comments are filled with: Links to virus sites, Advertisers, Self Advertisers, Spam, Copy and paste pics of dogs"
side effect benefit is the complete removal of any and all comments.
The comments rarely add anything of value to the video you are watching and most of the time end up being an argument about something totally unrelated, so for me they won't be missed at all.
However, YouTube have added an option to enable comments on a single video temporarily just in case you wanted to dive into the comment cesspool.