With the evolution of the Metro user exprience, the guys at Microsoft decided to take this one step further an make Visual Studio Metro-compliant. The entire UI was revamped - it is now flattened and is using a very restricted color scheme. You can see some examples of how the UI changed in the official announcement blog post.
The changes drew a lot of criticism from seasoned developers, but is the change really as bad as developers claim it to be? Not really. The layout of the general development elements (toolbars and panels) remained the same, and the icons are still somewhat similar to what was before. For me personally, VS11 with the light theme reminds of Photoshop CS4 (Scott Hanselman mentioned that here). With the dark one - Expression. Are a lot of people complaining about that? Not really. The simplistic color scheme actually makes the UI more "clean" and organized - it is easier to see the important components and avoid the extra noise. But that was already said:
In previous versions of Visual Studio we made use of boxes, separators, bevels, gradients, and shadows to create user interface structure and emphasis. One unintended consequence was that the combined effect of this ‘line work’ drew attention away from developer content. In VS 11 we removed as much structural line work as possible.
Another specific user concern is the fact that panel labels are very hard to differentiate due to their use of capital letters. In its current state, it indeed is a bit more confusing than it could be - the current font size and style do not facilitate easy recognition. This can be easily fixed by making the headers a bit larger and in bold. A UserVoice member - thenonhacker, posted a suggestion for some UI modifications that make Visual Studio 11 look a bit more Metro-ified:
UI changes are inevitable. Look at what happened to Office in just a couple of years - the new Ribbon UI was welcomed with some heavy criticism, but later on everybody got used to it. I expect the same to happen to Visual Studio 11 - the current heavy criticism should die out with more hands-on feedback.