I think the primary reason we have trouble talking about quality is we so often confuse it with value. Artistic quality is excellence in an established cultural tradition. That tradition has a form with a set of rules and expectations, a unique physics of engagement, a shared language; and from that tradition, excellence is expressed.
You do not need to like or value that tradition to recognize when its expression has quality.
An example: I don't know much about the tradition of ballet. However, I know enough to recognize excellent ballet dancers from merely competent ones because I have had enough exposure to the form. Some cultural traditions have very simple rules: others are more complex. It may be that complex cultural traditions require more but give more in return because their complexity provides a greater range of expression. But whether that is true or not, if you have enough exposure to a tradition, you are able to discern, even without being able to articulate exactly why, variations in quality.
This is a clarifying insight, and I feel it's a distinction missing not only from most published criticism but from much discussion of criticism. Instead, many critics, and many critics of critics (pretty much everyone else), traffic much more, often unconsciously, in the next kind of judgment:
Which brings us to value, which is a moral judgement, not an aesthetic one. Value judges what kind of work is important - theatre of social justice, devised work, Broadway, Indie theatre - and in doing so, also judges what kind of work is not important.
This is where the best critics and a lot of their readers part ways, because while a truly open-minded critic tries, as best they can, to embrace and understand as many traditions and sub-traditions as possible, and to evaluate them on their own terms, their audience is often most interested in what kind of experience they're going to get:
An audience that loves the tradition of experimental theatre begin with a set of values, and when experimental theatre validates those values, that audience is far more likely to believe the work is quality. An audience that believes theatre for social justice is more important than the classics immediately turns off when the curtain rises on a traditional production of Shakespeare. And so on.
When theatre does not conform to our values, it is very difficult for us to assess its quality. Why? I think in part because questions of value are so deeply connected with self-identity. Broadway theatre isn't just bad, it's everything wrong with theatre today! Theatres should only produce works by playwrights under 35! We should ban Shakespeare! Behind those firey calls for revolution is often, I think, a real fear that the work we're doing isn't valued, and so we must devalue work from other traditions.
And of course we should advocate for the kind of work we value, but in doing so, we should never confuse that advocacy with a clear-headed analysis of quality. The mediocre play with the beautiful process of international collaborators concerned with peace is just as deadly to experience as the millionth production of A Midsummer Night's Dream. So while I think it is worthwhile for us to talk about what kind of theatre we need to see, a cooler-headed look at how to make better work across all traditions is increasingly important.
I have to say, Good luck with that last aspiration, though the generalist critic in me applauds the principal. After decades logging time in front of all shapes and sizes of theater, I get pretty impatient with work that's not excellent, or doesn't at least possess some kind of excellence or original spark, and I find that I honestly care much less what kind of show I'm seeing--big honking musical, tiny site-specific solo show, whatever--than if it's any good.