How has it gone downhill?
Metafilter is like the American suburbs of the internet. Nice and tidy, but boring.
And full of anxiety and dysfunction masquerading as something else.
Shortly after cortex joined, submissions and comments which did not conform to a very narrow set of socio-political viewpoints started getting censored, and these days there is pretty much no room for controversial content.
Sure, I can see that the site would look highly civil and somewhat interesting to a new user coming from, say, Reddit, but to me the fanatic censorship has turned the place into a boring echo chamber compared to what it used to be.
That's a lot like trying to dispute the effect of Donald Trump's presidency on the US by noting that he has been an American since he was born.
Eh, I don't think that's really a fair characterization. Yeah, sure, they delete dups, etc., but no one's complaining about that. The mods also delete stuff from an ever-increasing (but ill-defined) list of topics that "Metafilter doesn't do well," which basically means anything that could upset the cool kids or other favored groups.
There's a lot of hype about Metafilter's famed moderation, but all it's really done has caused the site to reach a different failure mode. Rather than discussion, you get a bunch of commenting that's bracketed by passive aggressiveness and anxiety, carefully moderated into an illusion of civility.
That's the standard justification, but it leaves a lot to be desired. For one, IIRC, even brand new topics have been moderated away because the mods judged discussion might not go in the "correct" direct (for lack of a better word). Two, avoiding disagreement to the degree Metafilter does is a recipe for tension and dysfunction.