First, HomeKit in iOS 9 will no longer require an Apple TV as a hub to control your HomeKit accessories. Remote access in iOS 9 now works over iCloud, and all traffic is encrypted. Vendors have access to this new protocol for free and Bluetooth LE accessories makers can also send notifications to your device.
New in iOS 9 HomeKit is also the custom event trigger feature. In iOS 8, developers were limited to either time-based triggers or geo-fence triggers -- that is, developers could only activate an accessories based on location or a time. Now developers can trigger certain scenes (activate or deactivate accessories) based on a set of different user activities. For instance, you can tell HomeKit to turn on all the lights when you get home after 7PM, but not if its before 7PM.
Also new in HomeKit are four pre-defined scene types, which are common to most people's life, including getting up, leaving home, returning home and going to bed. These scenes will remain constant across accessories and Siri even knows the names of the scenes, so all a user has to do is say them into their phone to activate it.
Finally, HomeKit now supports the Apple Watch. You can now use your Apple Watch to trigger HomeKit actions even when you aren't carrying your phone. For instance, if you ever leave your phone at home, you can unlock your HomeKit-enabled door using bluetooth proximity or with the Watch app.
Even though HomeKit was just introduced last year at WWDC, Apple is clearly making lots of improvements to the framework as devices and accessories make their way on to the market.