A key theme of the updates to Mac OS and iOS this year is what Apple is referring to as continuity: being able to start a task on one device such as an iPhone and then continue it on another device such as a Mac. This notion was demonstrated at the WWDC keynote with the task of writing an e-mail on one device and continuing it on another, or going to a web page in Safari on a Mac and then picking up the same page on an iPad.
In principle, this could be one of the biggest and most useful innovations to the Apple ecosystem, and certainly received something of a rapturous response from the WWDC audience. But the devil will be in the detail: how extensively is this rolled out across the built-in apps, and what support do developers have to build this functionality into their apps, and how easily? I look forward to seeing more details...!