I think considering brevity, for its own sake, to be an important rationalist virtue is unlikely to prove beneficial for maintaining, or raising, the quality of rationalist discourse. That's because it is a poorly defined goal that could easily be misinterpreted as encouraging undesirable tradeoffs at the expense of, for example, clarity of communication, laying out of examples to aid in understanding of a point, or making explicit potentially dry details such as the epistemic status of a belief, or the cruxes upon which a position hinges.
There is truth to the points you've brought up though, and thinking about about how brevity could be incorporated into a list of rationalist virtues has brought two ideas to mind:
1. It seems to me that this could be considered an aspect of purpose-minding. If you know your purpose, and keep clearly in mind why you're having a conversation, then an appropriate level of brevity should be the natural result. The costs of brevity, or lack thereof, can be payed as needed according to what best fits your purpose. A good example of this is this post here on lesswrong, and the longer, but less jargony, version of it that exists on the EA forum.
2. The idea of epistemic legibility feels like it includes the importance of brevity while also making the tradeoffs that brevity, or lack thereof, involves more explicit than directly stating brevity as a rationalist virtue. For example a shorter piece of writing that cites fewer sources is more likely to be read in full rather than skimmed, and more likely to have its sources checked rather than having readers simply hope that they provide the support that the author claims. This is in contrast to a longer piece of writing that cites more sources which allows an author to more thoroughly explain their position, or demonstrate greater support for claims that they make. No matter how long or short a piece of writing is, there are always benefits and costs to be considered.
While writing this out I noticed that there was a specific point you made that did not sit well with me, and which both of the ideas above address.
Similarly, the number of words in our posts and comments is a cost for the things we actually care about (e.g. legible communication), and all other things being equal, we'd prefer this number to be as small as possible.
To me this feels like focusing on the theoretical ideal of brevity at the expense of the practical reality of brevity. All other things are never equal, and I believe the preference should be for having precisely as many words as necessary, for whatever specific purpose and context a piece of writing is intended for.
I realize that "we'd prefer this number to be as small as possible" could be interpreted as equivalent to "the preference should be for having precisely as many words as necessary", but the difference in implications between these phrases, and the difference in their potential for unfortunate interpretations, does not seem at all trivial to me.
As an example, something that I've seen discussed both on here, and on the EA forum, is the struggle to get new writers to participate in posting and commenting. This is a struggle that I feel very keenly as I started reading lesswrong many years ago, but have (to my own great misfortune) avoided posting and commenting for various reasons. If I think about a hypothetical new poster who wants to embody the ideals and virtues of rationalist discourse, asking them to have their writing use as small a number of words as possible feels like a relatively intimidating request when compared to asking that they consider the purpose and context of their writing and try to find an appropriate length with that in mind. The latter framing also feels much more conducive to experimenting, failing, and learning to do better.
ever since i read your post on epistemic legibility i find myself thinking about this whenever i'm about to link a source. its resulted in me relying on citing sources much less frequently, and (i think) made my writing clearer and easier to follow.
i think this is a lovely post, and i hope the other points you've brought up in it will stick with me just as strongly as that has. especially the suggestion to do more to push back against posts that have substantial flaws... it really is so easy to skip over a flawed post without comment, and i don't like how often i do it.
i feel like the best thing i could focus on to improve with regards to this is trying to let go of what seems like a kind of maladaptive perfectionism. i'll often feel an impulse to write out a response that fully lays out whatever flaws i see, attempts to address the implications those flaws have on the rest of the post and the conclusions that it draws, all while anticipating and answering responses people are likely to have. that might be worth doing sometimes... but its not realistic as a typical way of addressing this kind of thing. getting more comfortable with lower effort responses to flaws in what people are saying feels like it should have high +ev.