The problem is that it defaults to "Everyone on Google+" For those of you mystified that this is pissing people off, it's because not everyone on Google+ wanted a Google+ account precisely because Google is shoehorning it into everything! "Just opt out" misses the point; some of us don't want to have to dig around in settings for a service we didn't want to opt out of stuff like being put on a spammable directory for strangers.
If it defaulted to "Circles" I'd have a hard time complaining about it; people who don't want Google+ don't have Circles. But as it stands, this is "we're quietly opting you into spam." (and for those who say it won't happen, I've already gotten more YouTube spam since the G+ rollout than I have over the rest of the account's lifetime combined, and my YouTube account is old)