"Anschluss", in the context of web services, refers to the poorly-conceived merging of accounts between YouTube, Google+, and other Google-owned properties -- and, more generally, lack of control of how one's online identity across different web services is publicly announced and connected by those services.
lack of identity control
The main problem with the G+/YouTube merge was the fact that users were given little to no control over which of their identities should be considered the same, and essentially no control at all over whether to merge accounts in the first place.
In my particular case, this essentially resulted in having to delegate usage of one of my two YouTube accounts to Harena, while I controlled the other one -- where we had really wanted for either of us to be able to access either one (one was for uploading family videos and short excerpts from copyrighted works for use as commentary, while the other was primarily for my original material).
Worse, though, is that it had the potential to "out" people who needed to keep certain identities separate -- I don't know of any specific cases, but if for example a closeted transsexual or gay person was posting pseudonymously on G+ while uploading family-related videos on YouTube under their public name and identity, the merger could have caused hostile third parties to be aware that this person was closeted and either to attack them directly or to threaten to inform their family and/or workplace -- possibly resulting in job loss, being disowned, or being physically attacked by others in their community.
It would have been far better if G+ had treated YouTube accounts similar to how they treat G+ "Pages" -- ownership of a Page is not shown to third parties, and a Page can be controlled by multiple G+ accounts. It's a mystery to me why they didn't do it that way, but then again many things Google does (especially with regard to G+) are a mystery to me.
breaks comment flow
Another more general problem is that reshares of a video on G+ are seen on YouTube as comments on the video -- which means that the sense of "who owns the post" is completely reversed. If I'm using a video as commentary on something else, then my post is seen by YouTube users -- not without justification -- as being off-topic, and possibly interrupting the flow of commentary on the video, rather than as being a separate post of mine in which I set the topic.
If I've reshared the video from another G+ post, I might not even have seen the commentary I'm supposedly butting into.
A better solution would have been for G+ reshares to appear on YouTube as links, clearly designated as Google+ reshares.
This is based on only one incident so far (see this thread), but apparently blocking YouTube users on a YouTube reshare to G+ doesn't "stick". In that thread, I blocked user "lorenei" -- which you can see took effect because my next Mention of lorenei was not a link -- but then lorenei was able to continue commenting, and was no longer blocked from my perspective (which you can see is the case because when I Mention lorenei next, it is a link).