> Completely flat design has no sense of layers or depth.
Windows 8 suffers particularly strongly from this because it's got a lot of software that wasn't originally designed for the Metro style. Outlook (http://i.imgur.com/7fWEmvm.png) is one of the worst examples.
There are so many small things that would help give a hierarchy to the design, like using a darker shade of gray in the ribbon background and maybe the sidebar. Without visual cues it's a mess that you have to stare at to understand.
edit: It appears the reply buttons do squish like that on narrower screens though, but it's at around 1000px wide or less, so that's really a non-point in this day and age. Gradient issue isn't there though.
It is possible that some of those buttons are from add-ins like CRM, which haven't updated to the new look and feel. Visual Studio 2012 has the same problem.
Although this by no means affects your main point about the organisation/visual hierarchy.
The application in your screenshot doesn't suffer from a lack of gradients and drop shadows. The problem is the almost uniform treatment of every element of the interface. This isn't a problem with "flat design" ( whatever that is ) but poor typography. One can ( and should ) create a clear visual hierarchy through type and color without resorting to shadows to give it more "depth".
Windows Phone has a lot problems as well. I open the Photos Enhancer app for example and it's impossible to know what is clickable or not. You effectively have to keep tapping on text/images until something happens.
The whole flat design approach still needs a lot more thought.
Windows 8 suffers particularly strongly from this because it's got a lot of software that wasn't originally designed for the Metro style. Outlook (http://i.imgur.com/7fWEmvm.png) is one of the worst examples.
There are so many small things that would help give a hierarchy to the design, like using a darker shade of gray in the ribbon background and maybe the sidebar. Without visual cues it's a mess that you have to stare at to understand.