What were some of the biggest mistakes that Yahoo made with the running of this Answers site?

In my opinion, a couple of mistakes were: The lack of site moderation and the reliance on trusted reporters, some of whom took advantage of that reporting power. Yet many abusive posts and abusive users remained. 

Another mistake was not having a specific section where people could chat, play games and tell jokes, without being reported for breaking the guidelines. What's wrong with making the site fun? There was a time when trusted reporters were taking down so many questions by the hour in Polls & Surveys, that many users ended up leaving this site. You need users in order to generate advertising revenue. 

Variable 462021-04-09T06:07:50Z

The Trusted Reporters were not the problem (I actually was one for several years, with the power to remove a question or answer based on my sole report).  The problem was the Trusted Reporters who became self-appointed Hall Monitors, over-zealously policing the site for anything they deemed an infraction even when it was clearly not.  They became most active in the period circa 2012 to 2013, and drove away a huge number of good users, creating a vacuum that was filled by trolls.  And that was the snowball that started rolling down the hill.

However, official site moderation was impractical.  At its peak, questions were rolling in by the second, receiving 5 to 15 answers, plus comments, plus feedback on Best Answers...it would have cost millions to have paid moderators.  A simple control on the Hall Monitors would have been more effective.  They were also Point Accumulators so I advocated that every over-turned report would cost them 5,000 points.  When they'd lost 50,000 points, they'd lose their Trusted User status.  THAT would have made them a bit leery about reporting some harmless chat or a simple "No" answer to a Yes or No question.

If not controlling the Hall Monitors was a big mistake, another one was the 2014 Green-to-Purple format change, and all the other changes that went with it.  It was widely despised but Yahoo would not, or could not, undo it.  And that's when the snowball really started to pick up speed and volume.

Finally, in the period 2015 to 2016 or so, the site was basically left hacked open for months at a time, for all sorts of odious content that no amount of reporting would remove.  I had been a regular user since 2008 but that's when I switched to a very occasional participant.  You could feel the end was coming, but I am a bit surprised it took 5 years to get here.  It's almost like Verizon forget this was even part of the purchase.
 

Jimmy C2021-04-09T05:29:38Z

I agree. Lack of proper moderation has  been a huge problem. There are lots of obscene and insulting posts on here, but at the same time my answers have been removed many times just for agreeing with a question, stating simple facts and even quoting from the Bible. 
Some of the rules are petty and silly. I lost appeals because the rules state that you cannot include a question in your answer, and you cannot agree with just the word yes. 
Common sense all around would have been nice. Fun is always good too. 

I Z2021-04-09T05:29:25Z

not letting my points be turned in for cash prizes didnt help much either