I agree that the card catalog should be a way to find databases by location, title, author, keyword, etc., but it is almost useless. The only time I have ever found a database by searching the card catalog is when I already know the name of the database. Forget that.
However, the "explore by location" function can be useful.http://search.ancestry.com/search/
Scroll down to US map.
For the US, click on the state. State resources are shown by category. On the right side, you can drill down to county level resources.
There are also tabs for UK, Europe, Canada, etc.
Is this method completely correct? I doubt it. But I have found it quite useful.