Waste #2: Extra Features | Agile Zone
Our first best weapon against extra features is a short feedback cycle. Frequent product demos will expose features that we're working on that our customers no longer believe will give them a competitive advantage. Even better than frequent demos are frequent production deployments. Getting the software in the wild on a regular basis and then tracking feature usage can easily expose features that are not needed. Removing features from the system will reduce the complexity, maintenance load, and likelihood that things will go wrong going forward.
Our second weapon against extra features is a healthy dose of "YAGNI." YAGNI stands for "You Ain't Gonna Need It." This phrase represents one of the original principles of eXtreme Programming, that of only adding functionality when it is necessary to meet a clear and present need of the customer.
Wikipedia's article on YAGNI  provides the following useful summary of the disadvantages of extra features:
* The time spent is taken from adding, testing or improving necessary functionality.
* The new features must be debugged, documented, and supported.
* Any new feature imposes constraints on what can be done in the future, so an unnecessary feature now may prevent implementing a necessary feature later.
* Until the feature is actually needed, it is difficult to fully define what it should do and to test it. If the new feature is not properly defined and tested, it may not work right, even if it eventually is needed.
* It leads to code bloat; the software becomes larger and more complicated.
* Unless there are specifications and some kind of revision control, the feature may not be known to programmers who could make use of it.
* Adding the new feature may suggest other new features. If these new features are implemented as well, this may result in a snowball effect towards creeping featurism.