Xcode 4.6.x had already some sort of flat design. They removed the colors and everything was flat and grey but at least it had some sort of depth. Look at these screenshots:
Now look at the new Xcode 5.0 flat design. I don’t see an improvement. It looks like an old fashioned GUI of the 90’s. It’s even hard to understand what the icons mean. It’s so abstract that someday we’ll need an ideogram dictionary to understand what those icons mean.
Now look at a very old version of Xcode 3.2.x and tell me which one do you prefer. Honestly I don’t see an evolution in the look and design from colorful and self explanatory icons to cryptic flat ones. Flat design is not bad and it doesn’t even matter if it’s flat or not. The point here is that an user interface should be self explanatory, an icon should be worth a thousand words.