Am I the only one who doesn't love the whole flat trend? It seems like once the novelty wears off, it becomes bland more than anything else.
Overall, the core of flat UI which is : solid shapes, contours and hierarchy are good elements to have. But, its an extreme. Gradients, shadows, highlights,etc... give UI elements dimension and help them to occupy space, there has to be some of those pieces to help users distinguish between different types of elements.
I am personally not a fan of this trend either. But, I consider it a trend of extreme minimalism with no middle ground which will not work well in the wild. If you look at it, very few products or companies are able to utilize it successfully.