Design is a bit like art…everyone has their own opinions and preferences. Its never easy to design for a website or mobile app and in my experience it is something that constantly evolves up until the point of release. With the fairly recent release of Apple’s iOS7 and the increased popularity of flat design, it makes designing and altering a design quicker and easier than it was previously. i.e. making gradients work correctly and consistently on all browsers and devices.
The importance of design is not just about achieving a modern style (if that’s what is trying to be achieved), it plays an important role in accessibility too. Font size, colour and type, UI colour schemes, app layout, format, padding, whitespace, contrast etc are all important aspects of ‘design’ but also play a part in making sure users can actually use the app comfortably and intuitively.
A benefit I see from flat design also improves performance, less requirement for images. Creating assets for retina and non retina displays for both phones, phablets and tablets can really put pressure on the application to find and output the correct assets as designed.
This link from Smashing Magazine is a few years old now but still has some good examples of bad mobile design. Or “overdesign” as they like to call it. The article mentions how overdesign is a waste of time and that users like familiar layouts and things like buttons, headers/footers and sliders should be uniform and at least be familiar in design to the operating system.