Firstly, it would not solve this problem since Valleywag stories tend to get lots of upvotes. Indeed, it would encourage posting link-baiting stories, since they also tend to be karma-baiting stories.
Secondly, it would stop people from posting borderline, but valid stuff. I often don't post things I'm unsure people will like, but many times posts that seem borderline to me end up getting loads of upvotes. With a karma cost for posting, people would simply not post these kinds of stories.
A karma cost for posting is a recipie for encouraging lowest-common-denominator stuff.