As a user and a video creator this would annoy me a lot. If they want to add a G+ button fine, but replacing a core feature? Unacceptable.
If you want to be pedantic then sure, they removed a core part of Youtube when they removed the rating system, but in reality the system evolved. 1 - 5 was (at it's core) a way to rate videos: like / dislike is (at it's core) a way to rate videos, the interface and finer points are different but it's still the same thing overall.
You somehow boiled it down to a discussion on the lack of a "thumbs down" button. I think you're off track here. YMMV.
I could like videos on Youtube without the risk of them appearing anywhere or being used for anything. This has changed. And frankly it sucks.
YouTube is a social network in itself (at this point a much more powerful one than Google+) so they have to respect its users and not try to force them to signup for the other network.
I don't personally vote on Youtube videos but I can understand how a video publisher might be upset that he's suddenly going to be getting fewer likes on his content.
Again it seems this was just a test, so considering the negative user feedback I certainly hope it stays just that.
Which imo is looking increasingly likely.
Vimeo is one competitor which seems to already be growing like crazy for example.
Youtube has lost their total dominance. Youtube is no longer the be all and end all of online video. Vimeo started off capturing business videos from youtube as they looked more professional, now they're capturing the indie filmmakers.
I said they need to be careful about not pushing YouTube into MySpace territory, not that they will. Nor did I say Vimeo are the next YouTube. You could see the video market fragment as it's not all or nothing as Social Networks are.
And who cares about the quantity that gets uploaded to YouTube, you know most of it's long tail junk. It's where the viewers go, not who has the most crappy video of their dog rolling around for a tasty treat.