An image board should be as verbose as possible. A thread should

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An image board should be as verbose as possible.
A thread should be a giant text wall.
In fact most of the thread that you read should be useless and just there to expand your contents and make your thread look bigger.
Verbosity means less people will read it therefore have a better chance at a high quality experience.
Anything less than 5 lines is a thread not worth reading and if your not practicing your typing skills on 4chan then you might as well be wasting your time.

In some ways verbosity is equivalent to spam since the message is so diluted in superfluous information that the reader has to be so invested in the author's first line to continue reading and come up with a proper response.
In essence, spam is justice.