Question: I know that the default answer is "it depends", but I want to know the feature list you are working from in developing a good application.

I'm particularly interested in features that need to be cared about at design time because adding them late will cost us a lot.

**Please list one feature per answer so we can vote for the most important features.**

Any extra info, like your own war stories, will be appreciated.

I ask this question to enhance my application from day zero, and I think I'm not the only one with this target, so please write whatever feature YOU think should be exist in a good application.

Answer: 
**Mouse-free operation**: To whatever extent feasible, the application should facilitate the user interacting with it without the use of the mouse. (File under Usability, Accessibility.)

This doesn't necessarily mean going to great lengths to support mouseless usage; more just refraining from forcing the use of the mouse where it shouldn't be required. For example, don't defeat the default form-submission behavior when pressing enter in a Web form unless you're going to do something more useful with it.
**Regression testing:** the ability to easily, routinely verify that key functionality elements continue to perform as expected as the system is modified. (This is an more specific element of 'extensibility'.)