Thank you for posting your feedback. We really appreciate it! We want to make the forum as user-friendly as possible.
I've looked into the posting length requirement. Fortunately, the setting is customizable. I also looked into why the default setting is set to 20 characters. The reasoning behind the minimum requirement is to encourage discussion.
But, I agree with you. I like to comment back with just a thank you. I've reduced the posting requirement to 10 characters. This should allow you to post a simple 'Thank you!' (with punctuation) as a reply.