You can customize your UI. Organize your icons, change your wallpaper. Apple has never given you the ability to "deeply" customize the interface. (Nor does WP8. And Android can become quite sketchy when you start doing wholesale changes to its "design" - usually you have to flash a different ROM to have such customization options; no thanks).
Jailbreak if you want that.
And just how much customization do you wanna do?
Well, android allows way more customization than iOS does.
iOS gives you the very basic. Wallpaper, icons placement.
WP8 is in the middle. You can change tile sizes, placement, wallpapers, themes of the phone to specified shades.
With iPhone they are all pretty much identical. There isn't much individualization of the phone.
And yes, Jailbreaking it does give one many of those customization options, but not everyone jailbreaks.
I'm not saying it's a must have on the iOS, all I'm saying is that Apple could've avoided this whole redesign of the visuals if they simply let developers create "Themes" for the phone. People would get "fresh" looking OS at there own will. And i'm sure there will be TONS more money made on it on the AppStore. It would free up Apple's Engineer's time to do more value added upgrades to the OS.
Just food for though.