Spam doesn't have to be just bots trying to get you to their site - any post that is unwanted, duplicative, and generally made just to boost post count is often called spam too, and I think if you encourage users to post, you'll end up getting them just posting for the sake of posting if you're not careful.
You could, you know, try looking at the link I provided above because it might be useful... and likely to work on 2.0 RC3 without significant changes...