The good thing about flat design is that people with terrible design skills like myself can now create sites using just rectangles and "background-color: blue;" and it will look modern.
The real lesson is: "flat design" doesn't affect your ability to design things at all, it's all in your head.
There fixed that for ya.
http://inspirationhut.net/wp-content/uploads/2013/08/23.png http://maxcdn.webappers.com/img/2013/03/flat-ui.png
Sure, color is an important tool at the disposal of a designer. But so are typography, layout, proportion, symmetry, hierarchy, contrast, and affordance. And those are just the static things! Designing good interactions additionally requires consideration of speed, motion, order, and timing.
Good design takes into account these qualities whether it's flat or not.
Thats not what Flat UI is about , that's what graphic design is about.
Flat UI is a gimmick,and a fad. Non designers like it because there is nothing to it, anybody can make something look "Flat". It became an excuse for poor design skills, and pushing generic templates as design work.
A large part of that, though, is its newness. Not a lot of developers are the cutting-edge, style-creating types, and the ones who aren't are still feeling things out.