This can easily turn in to a whack-a-mole game in the same way users bypass censored words.
In my observation, setting a min char limit will contribute to more garbage post content, such as:
* Quoting the OP or any other post in the thread, just to satisfy a minimum.
* Padding a reply with multiple smilies.
* Adding gibberish / random keyboard strokes.
* Adding a copy-past of text found elsewhere on the page, such as even adding the warning about "minimum of characters required" to their new post content.
A solution I have found to work is to lead by example:
* Create quality topics.
* Reply with quality content.
* Have moderators, and use them.
* Present your moderators with some guidelines.
* Present your users with some expected guidelines. (try to use the TL;DR format)
* Remove unwanted/undesirable posts -- Post moderation.
* Praise users who create quality content.
* Praising the intellect of your forum members, to encourage like-wise from their peers.
* Post insightful comments, or comments with links to additional sources for further reading.
These are off the top of my head. If anyone else has further comments on how they've dealt with similar issues in the past, please do share