Google provides a list of HTML improvements. Since forums are user generated content, the first post in a thread doesn't always include a suitable description. Some users will use the first post as a continuation of their title. I've setup up guidelines for users and best practices, but that doesn't help with older posts and folks who don't read guidelines.
I'm curious if others are experiencing similar problems and what actions they've taken to correct them?
I wonder if some sort of custom coding would add the second post content up to 155 characters. This way, if you have a short first post, the meta description would pickup part of the second post. This might eliminate short meta description problems. Google will create their own if they don't like ours, but at least this would reduce warnings and red flags.