The quality of commentary on a public comment platform has much more to do with a given community's moderation and social mechanisms than the username that appears above the comment. YouTube is a mess because it's a hit-and-run system, there is no social obligation, you just type what you think about the video and never look at it again. Additionally, YouTube has a short length limit on comments precluding in-depth discussions, and YouTube's audience is very varied and primarily non-specialized.
There are many public messaging systems online that have flourished and maintain a high level of quality even though they allow pseudonyms. Hacker News, for instance. I can think of five or six high-quality messaging systems wherein I have actively participated off the top of my head that were not flooded with YouTube-esque idiocy -- the name policy really has very little to do with it, I think.
Google+ is completely controlled by its users; you must explicitly accept any author's content that you wish to view and you can block and explicitly reject any author's content that rubs you the wrong way. It is a fundamentally different model than YouTube. In this case, people are generally only going to accept you into their circles if they know who you are at least vaguely, whether the displayed name is "cookiecaper" or "Jeff Cook". I just don't see the point in even having the argument regarding Google+ when it has to be explicitly regulated by the user anyway. Facebook likewise.
I think the real motivation here on the part of both Google and Facebook is to discourage the use of multiple identities so that it's easier to target advertising.
Pseudonymity is still possible, though, so I'm still not sure why this is such a major deal, because you can just go create a Google Account with whatever plausible name you can dream up. You can then reveal your identity to the persons you'd like and everyone else knows you as generic John Smith. In fact, I do this on Facebook, and I presume that many others with an interest in pseudonymity do so as well.