Understanding and gaining empathy with the people that use your product is the key to building successful things on the web. All design starts with an evaluation of users and the expectations of how your product will solve their problems.
Articulating how your product works without any of the burden of the visual style is a good way to make sure your product can grow as design trends change over time. Having a well planned out core to your product is vital in long term success.
Your product not only has to work well, it has to look great. The design of your product needs to respond quickly to the changing enviroment of desktops, phones and tablets. Build one product that works on every device to keep your users engaged.