Flat Design is not a ‘style’ or a ‘trend’. It simply does not exist. Let me explain:
Once upon a time, before the Photoshop layer style was invented, there was a thing called ‘graphic design’. It had styles, or disciplines. Swiss Style, for instance, could be referred to as a ‘style’.
Then as interactive or ‘digital’ design became more ubiquitous, designers applied a variety of effects, such as embossing, textures, drop shadow and lighting, to create a kind of nostalgic ‘American’ style design. This sat nicely with the practise of using Skeomorphic motifs and symbols to create a nice cosy feel to things. This was championed by iOS, which, based on Mac OSX, was based heavily around the Desktop Metaphor.
As happens in design, things change quickly, and the bold introduction of Microsoft Modern UI for Windows Phone (formally Metro) based on the principles of Swiss Graphic Design, gave users a new minimalist look, devoid of any superfluous styling, relying heavily on sans serif typography, giving the content a refreshed basic modern aesthetic. Facebook Home used this approach, as did the new Yahoo Weather app and Google properties.
Gradually the interface design crowd caught on to the concept that great content based design does not need all the glossy veneer. More and more designers design creating simple designs based on the general principles of graphic design, space, proportion, grid, typography and colour.
This back to basics movement is not to be given a term such as ‘Flat Design’, or ‘Flat Style’. It is simply ‘design’. And it is not a style, as that suggests that everything else is ‘unflat’ which is not a style. It is not a feature or a choice to be selected from a menu of design trends, “would you like it Flat?” is never to be a question for the client.
Following trends is always a bad idea, especially in mobile. This is most evident with the release of iOS7. As always, something is not ‘a thing’ until Apple bring it out, and now flat design is officially ‘a thing’.
The smart guys stopped making shiny bars ages ago, and designed clean, device-agnostic visual solutions, with almost no 3D or texture. When an OS is refreshed, only new interface paradigms should be updated. Your app design should stand on its own, it should not all of a sudden look out of place, old fashioned, a visual anachronism, because someone changed the OS visual style.
Good design is not a set of features or a trend based discipline, all good design comes from the same principles that have been around long before the systems that we design for today.