Flat design is the current trend seen in Windows 8, Apple’s IOS 7, and a number of color-blocked websites. I like flat design a lot as it fits with my own aesthetic of simplicity and minimalism. It forces the designer to focus on typography, shape and color contrast to maintain interest and usability. Many designers are using bright crayon colors along with it but there’s no reason more muted colors couldn’t work. It’s all in how you place them for proper contrast.
Flat design also makes responsive design easier with simpler designs that flow more easily between screen sizes and smaller file sizes.
When this theme went live, it was completely flat. As I’ve gone back since then and added back a couple of faint borders and subtle shadows. Not only do I find the result looks better but they also aid in usability by making it easier to distinguish between elements. Everything in moderation, even minimalism.