For the Generalist badge, the requirements are:
Provided non-wiki answers of 15 total score in 20 of top 40 tags
What defines what these 40 top tags are?
I assume it is just the tags listed under
Tags -> Popular, but I wasn't sure about this.
Exactly what it says on the tin: the 40 top tags are the 40 first elements in the tag list. That list is ordered by the number of questions decorated with each tag, descending.
Also note that the
Generalist badge won't be available until all of the 40 top tags decorate 200 questions or more.