A former boss used to say, "The surest way to starve a dog is to put two people in charge of feeding it." If people aren't individually accountable, they often assume someone else will take care of a given task. A potential challenge with the powerful workflow capabilities in Drupal core is that they don't make anyone individually accountable.
This module allows individual users to be set as Editors (can edit or approve content) or Reviewers (can only moderate content) at specific stages of your workflow. A submodule can also allow specific users to be automatically be added as editors or reviewers.