Unified UI Framework -- The new, more attractive Android UI is now unified across device form factors. Android developers can now write for Android devices as a whole rather than phones or tablets separately. Finally.
Social API -- Android's Social API provides developers with a way to store social data--contacts, profile information, status updates, and photos, for example--from multiple social networks. Social networks may or may not provide that information. If they do, the user must grant permission. A new People app utilizes the API to set up contacts as social connections.
Calendar API -- This API simplifies the process of adding calendar services to apps. It allows apps to add and manage events, alerts, and reminders through a shared database. This could be useful, for example, when creating an app to make restaurant reservations or determining whether friends are available to attend a certain event.
Visual Voicemail API -- The Phone app in Android supports new visual voicemail features, including the ability to access incoming messages, voice transcriptions, and audio files from multiple voicemail service providers. The Phone app can be integrated with developer apps to allow additional manipulation and organization of the visual voicemail inbox.
Android Beam -- The Samsung Galaxy Nexus and other NFC-enabled Android phones can use Android Beam to share information about their apps with each other when the phones are in close proximity. By bringing one NFC-equipped Android 4.0 device within a few centimeters of another, a user could exchange contacts, configure a multiplayer game, initiate a chat session, or share an Android Market link, among other things. Android Beam can switch over to Bluetooth for large data transfers; the advantage of Android Beam is that it doesn't require manual device discovery or pairing.
Face Unlock -- Facial recognition is often deemed to be creepy, but Android 4.0 uses the technology as an optional way to unlock the screen. Users can also fall back on PIN entry or pattern tracing to unlock their devices.
Enterprise Enhancements -- Android 4.0 comes with a new VPN API to make it easier to create apps that rely on secure connections. It also adds the ability for administrators to remotely disable the camera on managed devices, a necessity in certain kinds of work environments.
Hardware Accelerated 2-D Graphics -- Android 4.0 supports accelerated 2-D operations like scaling and rotating, as well as accelerated rendering of UI components. Speed means happier users.
Improved Media Effects -- With hardware 2-D support come various new camera features that can be accessed in apps. These include continuous focus, image zoom, custom metering regions, and face detection. Hardware acceleration also brings advanced filtering and image transformation effects that can be used on still or video images.
These and many other features await developers in iOS 5 and Android 4.0. If you're writing mobile apps, make sure to explore the documentation thoroughly. There's a lot more to see.