I don't entirely agree with everything here. The Keyword density doesn't seem to be a factor but I would say it is more about keyword placement. Place Keywords in Title, H1, H2, H3 and Bold tags. Also place keywords in anchor text of links and alt attributes on images. Next, Yahoo seems to love all links and quality doesn't seem to be an issue. Lastly, Yahoo doesn't have a "sandbox" it can just take longer to get indexed and ranked. Yahoo doesn't update the SERPs several times a day like Google does.