Yahoo has fairly recently updated its guidelines. You can read the full list at:

http://help.yahoo.com/help/us/ysearch/basics/basics-18.html

I thought that a few points were of great interest to you because we have discussed these issue in the past when describing the best way to future-proof your site against search engine algorithm changes.

Here are some of the new guidelines:

Pages Yahoo! Wants Included in Its Index ________________________________________

1. Original and unique content of genuine value

2. Pages designed primarily for humans, with search engine considerations secondary

3. Hyperlinks intended to help people find interesting, related content, when applicable

4. Metadata (including title and description) that accurately describes the contents of a web page

5. Good web design in general

It seems content is now King for Yahoo, and unique content at that. As I have always told you, develop webpages with unique content and your sites will do well.

Here is a summary of some of the guidelines Yahoo makes on what not to do. I have added my own comments in brackets afterwards:

1. redirects (targeting smart pages and similar cloaking techniques).

2.Pages that have substantially the same content as other pages (pages generated by page generators that substitute one keyword for another).

3. Pages in great quantity, automatically generated or of little value (data feed sites that are built in a day by someone wanting to put out thousands of pages quickly).

4. Excessively cross-linking sites to inflate a site's apparent popularity (webmasters who cross link all their sites in an effort to rank better).

5. Pages built primarily for the search engines (highly optimized pages).

Read the full set of guidelines for yourself as they offer a great insight as to why your site may have been dropped by Yahoo.