The key point is flat tends to remove visual affordances. E.g. The iOS6 video player has a bright blue 'done' button. A quick visual scan finds that button immediately. iOS7 foregoes the button for just the text, so now it looks like every other UI control, requiring the user to now read instead of scan. Yes, it's small, but I've noticed that since using the iOS 7 betas I feel a little less connected to my phone for lots of small reasons like this.

Regarding your second point, it is still quite easy to figure things out. But, I'd contend that it is harder than before. I just wish we could have a balance between extreme skeumorphism and flat designs.