Overall, iOS has the richest apps, as well as the broadest selection. Android has typically less-sophisticated apps and a decent selection. Windows Phone has the least-sophisticated apps on average and a small selection.
iOS's multitasking dock lets you easily see which apps are running and switch among them, and Android 4.1's Recent Apps tray does the same with a more visual punch. Windows Phone 8 has no equivalent, so switching among apps means finding them on the home screen or the apps screen, both of which involve lots and lots of scrolling if you have numerous tiles and apps. You can arrange the app tiles on the home screen where desired, as you can in iOS and Android, to help limit the back and forth. But you can't set multiple home screens, as you can in iOS and Android, to group your apps, nor can you create app folders as you can in iOS. All of this means that finding and switching apps in Windows Phone 8 takes more work than it should.
Android and iOS have long offered a notifications capability that apps can use to keep you updated on status, and iOS adopted the notification tray approach pioneered by Android in iOS 5. Windows Phone doesn't provide such notifications; it expects you'll use the home screen's tiles to track what's happening. Likewise, iOS's App Store app shows you when there are app updates available, while Android's Google Play app has a list of apps with available updates and lets you set apps to auto-update. Windows Phone has none of these conveniences; you only find out an app has an update when you open it and get an alert telling you to go to the Windows Store.
Like its predecessor, Windows Phone 8 supports dictation (pioneered by Android and added to iOS 6) and voice-based queries (pioneered by iOS 5's Siri and added to Android 4.1). The dictation capability in Windows Phone 8 works as well as in its competitors, and the voice recognition is more accurate than in Windows Phone 7.5. But its voice-based query is primitive. It supports only a few basic commands, such as "open application" or "call Bob," relegating all other queries to Web searches. Android and iOS both have a much richer vocabulary and can handle free-form inquiries such as "what's on my calendar?" or "how hot will it be tomorrow?" Windows Phone 8 has a long way to go to play in the voice game's big leagues.
For businesses, Windows Phone 8 adds a welcome feature: the ability to connect a device to a corporate app store, for easy dissemination of work-issued apps. Android has no such concept, and iOS's reliance on the use of OS X Server or third-party application management tools is more complicated for both IT and users.