I have been told by multiple other people at LessWrong meetings things such as “I used to post a lot on LessWrong, but then I posted X, and got mass downvoted, so now I only comment on Yvain’s blog”.

That's interesting, and is causing me to update in the direction of thinking that this is a real problem that resources should be devoted to solving. I think I know of one other person who's not you who has left LW because of downvoting. It's interesting how seriously we take the arbitrary numbers associated with our profiles & contributions. (I do it too.)

And it looks as though there are many people who have reported similar in this thread. Maybe talk to Kaj Sotala? Perhaps he is privately reprimanding mass downvoters?

I do think this comment of yours was a reasonable downvote candidate:

Then why do I see reddit links to NOAA articles, every single month, with titles like: "May 2014 the hottest May since 1880. Four of the five warmest Mays on record have occurred in the past five years. May 2014 marked the 39th consecutive May and 351st consecutive month (more than 29 years) with a global temperature above the 20th century average."

Not because I think you are wrong about global warming, but because frequency of newspaper headlines seems like a bad way to infer statistical trends. Newspapers report on what's interesting, what their readers will read, what's unusual, etc. So news stories are not all that representative of what's actually going on in the world.