I think that the character limit is a good addition, but seems a little short. I don't mind, but it might not stop spam posts because 15 characters isn't much. Either way, nice addition.
It will stop people from going lol or cool story. I think thats a good limit because sometimes you dont need a really long post to get your point across.
It is a great addition to the forum to stop low quality posting but 15 may still not be enough to be honest. People can still easily add in a few extra words.