Being a heavy user of Share sheets in iOS 7, I've always felt that something needed to be done about the growing number of icons cluttering these menus, especially given third-party apps are allowed to add additional 'Open In' choices to Share sheets.
As part of its extensibility mantra, Apple has said that third-party apps are now finally allowed to install their own hooks to the multipurpose iOS 8 Share menus, making it a lot easier to add new sharing functionality system-wide.
But as installing these extensions is bound to quickly populate Share sheets, iOS 8 allows you to easily arrange Share sheet icons and even tell the operating system which extensions are available through Share sheets...