Ability to search on a tag.
I was looking for information about management of an organization and couldn't find keywords that would accomplish this easily. If I were able to use tags in search somehow this would make it substantially easier to find the information.
Searches on meta information can have tremendous overlap with other tech concepts and make it difficult to find information.
This can be implemented as "search tagged posts" on the tag page or search bar recognizing tags and refining search results to posts having that tag.