It's always hard to create a flat design version of something created non-flat before. I really thought that Apple will do it's best to preserve the functionality of it's own designs and I think it really did it at 99.9%. Those icons aren't the best they could do, but it will change in time for sure!
Personally, this was exactly like what I was thinking-- to me, some of those icons look like bad mockup-stage design. Some of those gradients, in my opinion, are pretty bad and do not work.
Well you can never tell this, until you see it on your phone. Most of the icons may look like a basic photoshop tutorial, but still that's the minimalistic approach of flat design.
You've got a really different view of something ( a gradient ) outside of it context ( the whole UI ) and the same element as part of a whole design. That wasn't pretty obvious to the great super-pixel-detailed skeuromorphic design, where each icon was a whole design, but I'm pretty sure we'll speak differently by the end of the year when we get that iOS and evaluate the change for real.
Take a look at their new demo paga [1] it looks pretty nice and flattened. I'm afraid of what will happen to the 3rd party apps, that will not get a flat-design update.
Even looking at their site, the mail icon drives me nuts. Personally, I don't like the cyan-to-blue because it looks odd that the gradient shifts so dramatically in hue. The Twitter icon, on the other hand, is fine because it's not really a shift in hue but in lightness.
It just feels like the like source is different for different icons (some go light-to-dark, others are the reverse), and it's driving my designer eye crazy.
