As for the design choices that Apple has made, I have criticism.
Notifications and widgets need to be smaller and need to have an edge to edge design rather than this Apple Watch rounded corner look.
Control center needs to be customizable, and it's look should stay the same, the new one wastes space at the edges and ruins the beauty of the previous one. I don't like how the new one isn't edge to edge, I think that what makes most of iOS 9 beautiful.
The 3D Touch animations for notifications need to be more like the ones we've had for peek and pop.
Give the rounded corners a while to settle in, you might change your mind. I didn't much like them at first but I think they're rather nice now, and the space wasted is minimal!
Check out this thread in regards to notifications and space optimisation...
The state of iOS's notifications, and the steps Apple needs to take