The use of exploratory testing along with unit-level test-driven development is asserted as potentially more valuable than automated acceptance testing in this article from Brian Marick.

Test execution beyond the traditional use of developer unit tests in the Agile community has been debated. Elisabeth Hendrickson has a great article shared on her blog here.

Dhaval Panchal defines and provides usage scenarios for capturing, using, and misusing velocity in this article.

Mike Cohn has provided another great article on how a team can write non-functional requirements in a basic user story format.

The author of this blog entry describes how user stories are insufficient for quality requirements (aka non-functional requirements) and Chris Sterling has this blog entry on how to use an “Abuse User Story” to describe the cost of not addressing quality attributes in a user story format.