If they had just said that 'Okay, your Google profile is now called Google+, and your YouTube account attached to your Google Profile is now your Google+ profile, and you can go to your settings to complete the migration', it wouldn't have been so bad. What they didn't do is offer anyone any reason to change.
All the benefits they hawked for YouTube's Google+ comment integration were actually such huge drawbacks that many top YouTubers turned off comments entirely. Unsurprisingly, YouTube comments had gone from the mindless, racist, misspelled prattling of the internet's ignorant masses to giant spam-filled, link-filled dumping grounds of (varyingly) penis enlargement spam and giant ASCII dicks, and YouTube comments went from bad to worse, a feat no one thought possible.
It really feels like the decision was made for the YouTube team by someone else (Page? Gundotra?), and the YouTube team just had to roll with it, with their input being ignored and the Google+ team saying 'No it'll be great, don't worry, you'll see, everything is better with Google+!'
To this day, people invite me by gmail address to threads, which I cannot leave unless I tie my Google account to a Google+ account so I can join the thread so I can leave the thread.
The entire thing was a clusterfuck from start to finish, and everything that they wanted out of it could have been accomplished by providing value, rather than extrinsic rationalization.