What are the must-have features of related content?

How many modules should make up its core?

What should be extensions?

Modules that should be modified/accessed to provide Related Content suggestions

Very well maintained:

http://drupal.org/project/taxonews

A module to generate blocks containing titles of node matching terms in a taxonomy vocabulary, as links to the nodes themselves, with automatic content expiration based on publishing date.

http://drupal.org/project/similar

A module that displays a block with the most similar nodes to the currently viewed one, based on the title and body fields.

Only supports MySQL-based sites. CCK is not supported for similar matching.