I've been writing book reviews online since, oh, jeez ... 1990? Give or take a year? I think I just frightened myself. I wrote very occasional, not particularly organized reviews on an internet bb at the University of Washington. When I graduated, I went to DEC, and did NOT start hanging out in Notes, because I was not a VMS fan; I did, however, start hanging out on rec.arts.books and rec.arts.sf.written, where I posted, er, numerous reviews. I've posted a few reviews to on-and-off personal websites (now consistently "on" since, beats the hell out of me but definitely 2000), but I started this blog in 2004ish, and it has been the primary home for the book reviews ever since (the repro project meant a lot of those reviews are on the site).
Throughout this entire process, I've been getting feedback from authors, overwhelmingly a positive, feel good experience, even when I didn't necessarily like the books they had written. It is unlikely I'll ever have a single moment in my life quite as exciting as when I went to WesterCon and lined up to get a book signed and the author -- known for lurking long before I was participating -- said, "Oh, YOU'RE [my first name]". Honestly, more memorable than bungee jumping, particularly that first moment or two when I wasn't sure if this was a good oh-you're-that-person or a bad oh-you're-that-person (it was good).
Author feedback here has generally been in the form of brief comments and even authors who have not been happy about what I had to say about their babies have been excruciatingly polite about it. Apparently that's not the way things have been going on Goodreads.
h/t Nate Hoffelder
From my perspective as a completely ignorant, non-user of goodreads, that sounds like a plausible explanation. Generally speaking, more moderation is needed until you have enough, and then a tiny bit more, to give people something to think about before they go demanding their own, idiosyncratic things.