Generally because it puts a barrier in front of annoying behavior.
Is it an insurmountable barrier? No. But why allow annoying behavior when you can discourage it?
That said it’s a forum level setting. Don’t like it, start a forum, change the setting to zero. voila.
Because it discourages posts like yours which contain no meaningful content 
It’s a fairly common thing to do, very rarely will someone who has something meaningful to say be bothered by a reasonable minimum character limit. Even in the rare case that they are it’s not a big deal to add a bit more, if it’s not worth doing then chances are your post isn’t really important.
This.
I’ve always hated minimum character limits. I run into them every day.
I think it’s especially annoying that there’s no indicator of how many characters that you have to type. You only have whether or not the “Reply” button is greyed out or not, and not all users will realize that it’s greyed out at first. Even then, if you click on the reply button it can bounce back on you if your post was mostly white-space
It’s annoying as hell.
The only thing I’ve ever seen these kinds of rules do is cause people to make posts like this:
Something somebody said
No. 10 char.
I can’t remember where I saw it, but I remember seeing a minimum character counter that started at “10 characters to go”, then “Just 5 more characters” and then counted down. It kind of solves this problem.
Do you mean StackOverflow comments?
Aw, looks like someone patched the workaround of bloating your body length with emoticons.
This issue is definitely forum-specific, so you’re essentially just asking @codinghorror why he decided to set a minimum for meta.discourse.org.
Since this is a configurable option, this is really a non-issue.
I own a forum where 3-character posts make sense and a post that contains nothing but “+1” is a community norm. I would simply change the configuration setting and move on.
A minimum character requirement could challenge people to come up with things worth saying or sharing, even if they didn’t have anything initially. I’ve never come across this minimum requirement before, but I know I often decide to respond to a conversation before knowing exactly what I want to say. I tend to discover new and interesting ideas as I reply.
I think there are two types of people: People who write text walls, and people who don’t. A minimum character requirement is a way for the former to make the later conform to their tastes. But the opposite-- a maximum character limit-- could also be useful to get long winded people like me to simmer down a bit.
Given the context in which it is made, I agree this first post is eloquent. I get the point.
But it’s got a bit of “exception that proves the rule” thing going on.
We now show the minimum number of characters required to post at the bottom of the composer as a status, in the same area we show the draft save status:
enter at least (x) characters
as you type it updates dynamically:
15 to go…
then it disappears when you reach the minimum. This also unlocks draft saving, as we don’t want to save post drafts until you’ve at least reached the minimum post length.
And as with so many other things in Discourse, minimum post length is configurable via the forum admin site settings, but it defaults to 20 out of the box.
A maximum character limit can be seen as a limitation on one’s ability to convey a very complex and long message. Perhaps they cold scale it down but that does not change the thought process of a user who just makes three posts in a row because they couldn’t fit all of their text they wrote into a single post.
I find it difficult to justify a very short post of a single word unless under special circumstances. The user should be creative enough to get around this limitation without as much as pressing a few extra keys. Perhaps providing a confirmation that they intend to only post a very short post? I just don’t see how the purpose of minimum characters is useful for the habits it is attempting to circumvent.
These kinds of details will be adjusted from community to community based on the rules and policies that are put into place. Discourse should just be able to support these needed customizations.
Already implemented, 20 is out-of-the-box … you can scale it up or down in your community as you see fit.
Even if it’s a configurable setting, it is still worth discussing whether it should be the default setting.
“It’s configurable” doesn’t mean it isn’t a stupid feature to begin with—especially if it’s the default setting. 
Why so pessimistic? 
We choose defaults based on what we think is
- safe
- sane
- likely to lead to Civilized Discourse within our Construction Kit
That said, if you don’t like the 20 character default, it takes about 10 seconds to change it as the forum admin in your Discourse instance. Change it to whatever you like, up to and including zero.
I don’t see how this qualifies as a “stupid feature.”
In my book, it’s a good feature. What we called for was to make it configurable, which has been done.
One of the things that a minimum-xter feature brings to the table is preventing empty or one word responses that don’t add VIP value to the discussion.
If your response to a post is going to be “+1” or “cool!”, just “Like it.” That accomplishes the same thing.
I am not particularly concerned with the one feature—that I dislike—I just have a problem if this is how discussions of features are going to be had in the future.
Why not discuss the merits of and experiences with the feature instead of leaving the discussion at “well, turn it on or off, if you don’t like it, whatever”?
no
So many gaming forums have idiots (at least idiots in my eyes, but I’m getting old) who abuse whitespace in posts to create a 5000px height reply, or just reply with “lol”.
By adding it as a default behaviour, think of it as increasing www based communications by a few IQ points.