4. Keyword Density is more Important
While search engine companies keep their algorithms top secret, it is widely documented that Yahoo puts more weight on keyword density. Placing your targeted keyword in your content is important no matter which search engine you are optimizing for. Yahoo prefers sites with a higher keyword density.
Like always, there is a fine line between using your keyword enough and going overboard. You may have to tweak your content several times to find a happy medium that is accepted by both Google and Yahoo.