What Makes a Good Changelog?

When you have to update your project's dependencies, you (presumably) look at the changelogs for each package to see if you can safely update. This experience varies wildly based on how well changes are tracked and documented (if at all). I'm looking to suss out what separates a great changelog from a mediocre one.

  - What bits of a changelog are most helpful / important? 
  - What's not helpful, but usually included?
  - How should the document balance skimmability / comprehensiveness / readability?
  - What projects have the most helpful changelogs?

Thank you!