only count posts that are greater than X characters in length (say 150 characters or more?)
A lower benchmark perhaps? I can ramble for England, but most folk manage to say in a few words what it takes me a couple of paragraphs to say!
OK, so you might not want to encourage more "pointless" posts, such as just a smiley or a countdown etc, but a post of a couple of words can make a point with as much validity as a rambling post like a Wannabe special... Easier to read too