From WatchOS2, the watch app extension will run on watchOS and therefore the app become much more responsive.
Digital Crown can control pickers natively. Custom controls are also supported via protocol. Coordinated Images are used for special design like like a clock ring.
More API available that in Storyboard will be available in programmatically. Animations are supported in WatchOS2
Related Session: Layout and Animation Techniques for WatchKit
Different style of haptic can be used.
Related Session : Designing for Apple Watch
There is a controller that recording voice on watch
Movie and short audio content can be played on watch.
native player controller for playing long-form or stream audio. App is not required to run in background
Alert API on watch for user confirmation. similar to AlertController
sms url to reply on watch
tel url to make phone call on watch
Add pass to watch and iPhone wallet
Related Session : WatchKit In-Depth, Part 1 and part 2
To make complications
Related Session :Creating Complications with ClockKit
Watch can directly access networks if the Phone is not present
Related Session :Networking with NSURLSession
New framework for sync the data and files between wath and phone
Related Session : Introducing Watch Connectivity
Core Motion can now used in Watch. Past records can be retrieved for the app and allows querying
Related Session: What's New in Core Motion
Watch can get location. Permission are synced with Phone. Once user approved the permission, phone follows.
Related Session : What's New in Core Location
Similar to Core Motion, Health Kit can be used in Watch as well. Watch can also start workout and retrieve related health data like heart rates
Related Session : What's New in HealthKit
Watch has its own keychain items. Consideration of data when the device locking or not is in related session.
Related Session : Security and Your Apps
Watch can retrieve directions from MapKit and used that guides user to destination.
Related Session : What's New in MapKit
Likewise, contacts can be seen in watch.
Related Session : Introducing the Contacts Framework for iOS and OS X
Access user's calendar via Event Kit