Speaking from my own point of view, the "anger" relates to aesthetics. Personally, I'm not angry, just reeeally disappointed: I.do.not.like. what they did with the
look of the OS. I
prefer the look of damn near everything in iOS 6. I find most of the iOS 7 design to be trendy and (obviously) polarizing:
- Icons-Hate ALL of them in comparison to previous versions
- Calendar: Too "plain text-y". And the red does not help.
- Mail: Again, too "plain text-y"
- Overall "cartoony" look. I prefer the skeuomorphic approach, which makes things more intuitive and reduces iconographic interpretation.
Things I
do like:
-Lock screen (great big "buttons" for passcode)
-"Bladed", translucent interface
- Parallax effect and OS animation
- Multitasking implementation and overall OS functionality
I believe that Apple users are very concerned about aesthetics (Steve was too-to the point of insanity). I do not recall such vocal dissention in recent Apple history with regards to a product's appearance (aside from Game Center green felt). There may have been, I just don't remember (probably because I liked
everything they did until now).
This is all personal and in no way means that you are wrong for loving iOS 7, on the contrary. (See my signature

)
But it bears to mention that at least some Apple users, even enthusiasts like myself, are going as far as calling an Apple product
ugly. Nobody is wrong here. It is simply, a matter of taste. I simply disagree with Apple's (under Jony Ive) taste here, for the first time ever. And for me, it's heartbreaking.