>How is "quality" defined on an imageboard (or in a discussion medium in general)?
Quality has a subjective aspect to it, as readers often associate a text's quality with its compatibility with their world views. For example, a fascist might say a very well-worded and sourced blog post is of low quality because it talks about Marx in a positive light. If we look at the matter in this way, quality is not only a factor of the material, bot something that varies according to the reader.
We might want to look for universal quality indicators. I certainly think of a few:
First and foremost, clarity of message. If a post is worded in such a way as to prevent easy comprehension it is necessarily of bad quality. I am not referring to texts where the poster uses unusual vocabulary or discusses obscure topics, but to failures to transmit information in a way that can be understood without a double meaning or with an obscure one.
Correct use of grammar and spelling are (as I see it) important even when looked at independently from the first factor. They indicate that the poster cares about abiding by the rules of communication, knows them, and considers the place of posting (be it the thread or board) worthy of the effort to abide by them. Despite being more of an indicator of the poster's quality, it is also related to the post.
Profanity should be considered an indicator of a lack of quality. One might argue that there are situations/threads where the use of profanity is acceptable (and sometimes encouraged); the way I see it, being in such a thread is likely already an indicator of low quality in itself, unless the poster is trying to rescue a thread that has descended into a flame war or such.
Needless aggression should also be shunned. To seek or fuel conflict, especially on the internet, denotes a tremendous lack of perspective on the part of the poster. Even if "the other guy started it", appealing to needless confrontation achieves nothing except entertain small-minded spectators. By remaining calm and polite one indicates one's superior control over themselves.
Politeness, as mentioned, can probably also be taken into account.
Formatting that is aesthetically displeasing, tiring the sight and making reading uncomfortable, can also denote a lack of quality.
As you see, most of these actually refer to the quality of the poster rather than the quality of the post itself; the two are likely very closely related. It should also be noted that the length of a post, in my view, has no weight in its quality.
>How can quality posts be encouraged and preserved?
Besides directly pointing out the positive/negative factors in a post, or chastising/praising a post in a general way, the best course of action is to create a context where users feel encouraged to make quality posts, and discouraged to make low-quality posts. This is achieved by only making quality posts ourselves, and carefully policing what and how we say things. This >>1418 post by the board owner is slightly related (if a bit try-hard on his part).
>How can knowledgeable posters be brought in, and what best induces them to share what they know?
In order to bring in knowledgeable posters we must first find them and then inform them of the existence of this board, i.e. good marketing. As for inducing them to share what they know, we go back to the previous matter of encouraging good posts by making good posts ourselves. People naturally want to share what they know and be heard, we must provide them with an environment where their ideas, regardless of how complex and unusual they may be, will be heard, appreciated and possibly discussed.
>>1756 makes a good point, but we should remember that we don't need a large amount of posters, just enough to keep the board alive. Determining the necessary number of posters for that is a whole other matter.
All of this being said, I do believe that this board is home to an unusual level of quality among both posts and posters. Of course, that just might be my unsubstantiated opinion, but if anyone else agrees it might make sense to study why this board displays such quality, and what can be done to mimic these circumstances in other boards.