The biggest mistake people make is porting existing features over as-is. This generally means reproducing complexity created to serve business processes as they looked years ago, which is enormously wasteful. Whenever you are asked to add a feature which represents a change to a business process, go and observe the process from scratch and look for ways to simplify it before implementing the code to support it. You will find that much accidental complexity in business processes actually comes from being forced to use the old software you are replacing!
