Flat design is good as long as there are no important visual cues lost what elements can be interacted with and which are just passively displaying information. I really do still have problems in iOS7 to separate passive text elements from interactive text elements (formerly called "buttons"). Examples are the contacts list or the famous shift-button of the onscreen keyboard.
Thankfully in OSX 10.10, buttons are still recognizable as buttons, and I like that less radical flatness much more then iOS7. Although, in the current state the UI looks a lot like a Gnome skin which tries to mimic OSX though, I hope that improves until release.