I'm pretty sure you can't just copy a mechanic and file off the names. Those mechanics are copyrighted material in copyrighted books. You can be sued. How litigious the publisher is may effect your results, and some might not care, but if you copy something that's against the law.
I remember when the open license came out for D&D 3.0, it was a big deal. People thought it was amazing that WoTC was opening up their license for everybody to use. Other game companies have since copied that, but there are companies that hold very tight grasp of their intellectual property, and you can't use it.
An example, I read a blog post by an author putting out a Superhero game, Wearing the Cape, based on his own series of novels. He originally was going to use one system. It's been a bit so I can't quite remember which engine, might of been M&M or Gurps. Anyway, he wasn't able to get permission to use the engine, or at least not at a price he coud afford, so he went with FATE instead. FATE is an open license game, so he was able to do it.