What was the subreddit that represented to you the best example of downspiral of quality? To me it was /r/dataisbeautiful
At first it was all about presenting data in an original looking way. In the end it was about pushing political ideas in your throat using a plain bar graph. It was not about sharing something interesting you found but about taking advantage of a captive audience.
r/mapporn for me. Originally it was for beautiful high quality, high-resolution maps - the standard was so high that I would have been scared to post anything myself unless I found something exceptional, but eventually it became mainly low-quality (and usually inaccurate) data maps that all get mass-upvoted for some reason.