Too often the style guide is confused as being a brand guide in design projects. The style guide document says that the logo is yellow and the suggested color palette along with that is black and the font Helvetica, that doesn't mean that the brand is the summation of all these design elements. The brand is not about the right combination of these elements. To often the company focuses on seeing beautiful design work and expect that it will fit their definition of cool, versus their rivals. The designers themselves can also be to blame about what a website or interactive experience can be. They sit down and produce portfolio worthy concepts that look amazing and really make those brand elements shine.
However, underneath all of that is the true brand of the company. The story that the company tells itself and it's consumers or customers of it's content. They should all be focused on making all decisions on building and supporting those messages. Once you pull back the curtain and see the client and their brand at its core then you can begin to make design decisions that help support those ideals. Until then you are only working on a fraction of the overall brand value you should be thinking about. Yes the logo is important and the style sheets valuable, but they do not get to the heart of the message and the direction things should move in. The visual aesthetic can be thought of as the language of the company the brand though is the message and meaning they support.
The great thing is once you get to the brand core then decisions become surprisingly easier. What should our website experience be? Look at the brand. What user experiences should we work on in mobile? Look at the brand. I think the concept is clear. The brand can be the driving force behind many decisions and help design teams focus their ideas and attention to the right ideas. The client is also about to understand an idea and it's true value if they to are reminded of their brand value and messaging.