One of the major problems with having what we would maybe call an “identity layer” like that is what becomes the verification of real personhood. Airbnb asks you to take a picture of your passport or your driver's licence. I'm not super comfortable with Airbnb having a copy of my passport when I logged in through Facebook and then 50 million user accounts' information has been accessed. There are major implications there that could be really problematic in terms of privacy and personal data security.
There are also questions about whether anonymity would actually fix some of these problems. Nathan—I forget his last name—he is is at MIT, wrote a really interesting review of the role of anonymity and whether it helps in cases of hate speech and disinformation. In fact, getting rid of anonymity largely wouldn't solve those problems, so that's potentially problematic as a solution.