Each new version of iOS and OS X creates more integration across Apple's computer and mobile platforms, with a clear goal of treating all devices as optimized instances of Apple's integrated environment. You see that with the adoption of iOS-style gestures and text-entry shortcuts in OS X, as well as in iOS 7's adoption of the App Exposé-like view of running apps. It's even more evident in Apple's applications: Mail, Calendar, Contacts, Reminders, Notes, Messages, Safari, Maps, iTunes (if you include iOS's Music, Videos, iTunes U, and Podcasts apps as part of the iTunes family), iBooks, iMovie, iPhoto, GarageBand, Pages, Numbers, and Keynote.
They work increasingly the same across Apple's full range of devices -- except when they don't.
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Persistent inconsistencies among the "same" apps between OS X and iOS -- and sometimes across different types of iOS devices -- are an ongoing frustration with users. Apple users experience the frustration precisely because the common apps and OS-level functions are almost the same. They see the Apple promise of a common computing fabric and get upset when they encounter a tear or hole in that fabric.
Sure, there's a huge gap between how Microsoft services and apps work across Windows 8, Windows RT, and Windows Phone. Google's services -- Maps, search, Gmail, Calendar, Drive, Now, and so on -- are similar across platforms but not expected to be the same, perhaps because Google doesn't face compatibility issues with a desktop version of Android or a mobile version of Chrome OS. In that regard, Apple's unification state is much better than its competitors'.
Still, Apple has set the bar for a highly integrated fabric, so when it fails, it's Apple fault. There are several such gaps in the fabric that persist even today that Apple needs to remedy.
Groups in Contacts. You can't create groups in iOS's Contacts, but you can see those created in OS X's Mail. You can't address an email to a group in Mail in iOS either, though you can in OS X's Mail. Both iOS and OS X should let you create groups and address emails to groups.
Complex calendar repeating events. I hear this complaint after every iOS release: Why can't I set up complex repeating events like the third Thursday of the month or every month on the 15th in Calendar? After all, OS X's Calendar can do it, as can Android's Calendar app. iOS's Calendar app should be able to do so as well.
Mail threading. OS X and iOS let you view email threads (aka conversations), but in OS X your conversation shows both what you received and what you sent, so you see the full conversation. iOS's conversation list shows only the messages you received, not those you sent.
Mail rules. OS X lets you create mail rules for sorting and responding to emails, including junk mail. These rules sync across your Macs via iCloud, but they don't run in iOS. Thus, when you're on the road and your Mac is off, all your mail pours into the inbox. Maybe background performance was an issue for running rules in iOS Mail, but now that Apple is switching to 64-bit processors and has made iOS 64-bit-native, its mobile devices could easily run these mail rules as well.