Well, I share your confusion. I looked at the guidelines and your site, and I don't see anything that is actually bad by their standards.

I do see some things that an automated system might consider bad:

Your home page has hundreds of links to the topics available, which are of such quantity and variety that it might be interpreted as spam links, just a mass of keywords covering the page for the benefit of search engines. However, I consider the design and layout to be very good for what you want to achieve, and it is very inviting to a first time visitor, which I am. My inclination is that if it comes down to a choice between changing your layout or keeping it and losing Yahoo, you might as well let Yahoo go.

Also a Google site: search turns up 140,000 pages. That is a huge number, though it's not millions or billions that some sites have. Still, it fits the pattern of a auto-page-generated scraper site. Again, if all your 140,000 pages are legit, I'd say keep them.

Those are the two things I saw that their algorithm might object to.

Whatever ideas you come up with for the possible reasons you've been banned, you might consider writing them, specifically mentioning your best guesses, and ask for a human review to prove that you're not a spammer or scraper. I have no idea if they'd do it, but it's worth a try. There's nothing to lose.

Edit: The free for all links directory is a likely culprit. Getting rid of that was good.