Adding a new feature is a very common task for developers, so we just want to agree on a very short lists of things that should accompany a new feature.
No new feature without a test. TDD rocks, every developer knows that. Adding tests when adding a new feature is a great way to create useful tests if they were missed/not written yet because you don't have to write many tests in a row but only one at a time. This should be enforced by code managers!
No new feature without Wiki documentation. Adding a new feature to the software is fantastic and enriched the functionality but if nobody knows it does not make much sense. Add a description of the new feature in the product wiki page what it is doing and how it can be addressed.