What are we measuring, exactly? It's hard to talk about without defining what we're measuring.
A stab at defining some quasi-objective metrics, or, How I think about social news quality:
Are all the good stories getting submitted? (Sometimes the real question is: Are there any good stories on a given day, at all? HN usually nails the stuff I would choose to read in my blog reader. There is good volume here.)
How many bad stories are submitted? Does it matter?
Are the good stories being voted up? (I'd say 'usually' for HN. Though some gems are missed by the upvoters. Bowkett had his example. There was an awesome PDF earlier today that wasn't voted up. I blame a shoddy headline there.)
Are the bad stories being voted up? (I'd say sometimes for HN. There was that bad bout right after TechCrunch. And lots of Ask YC posts that bore me make it to the front page.)
How good is the discussion? (HN is the best I've seen for a site of this wide scope. People are harsh on lazy English, good at ignoring trolls, good at pointing out logical fallacies politely.)
How easy is it to choose good stuff to read? (On the front page, points and # of comments are great indicators. Lots of entries means there's plenty to scan from.)
One consideration is that I come back to HN at different times for different purposes. Sometimes I'm bored and looking for some short fluff to tide over a compile. Sometimes I'm bored but energetic and trawling for something interesting to learn. Sometimes I'm in the mood to check out what other people think about some issue, sometimes I'm not. Sometimes I want to bust 3 minutes on a quick tidbit, sometimes I want to bust 30 minutes on a paper or good Gladwell article. I can't judge HN for my mood swings, only for how easy it makes it for me to satisfy the varied cravings.
All in all, I like HN just fine, and I encourage Giles Bowkett and people like jrockway below to vote with their back buttons.
Wouldn't it be preferable to be good at ignoring lazy English? If the lazy English comes with trolling, it's ignored completely; if the lazy English comes with sincerity, everything's handled politely.