What Modern UI design actually is
Since Google presented their new UI design, there has been a lot of discussion going on, on if and how Google took influence from Microsoft's Modern design language. Reading some of the discussions, I have noticed that a lot of people don't seem to understand what Modern design is actually all about, making the whole discussion pointless. A lot of people, on both sides of the discussion, seem to assume that it's all about flatness and color, but that's just part of the visual manifestation of some of the underlying design principles of Modern design.
You can read the Modern design principles here: http://msdn.microsoft.com/en-us/library/windows/apps/Hh781237.aspx
As you can see, Modern design is about much more than just flat colorful graphical elements. I think it's ok to compare Material Design and Modern Design and to look for influences one took from the other, but these comparisons would be much more relevant and interesting if they were not just about the most superficial visual aspects, but about the actual design principles that Modern and Material design are based on.
I wish I had the time to do some in depth research on this and write a post about influences, similarities and differences between Material and Modern design, but maybe somebody else can do that ;)