"Hey Siri" support and possibly wireless charging case alongside AirPower charging mat.
Bluetooth LE Adapters Don't Enable OS X Yosemite's 'Handoff' on Older Macs
In discussing the limitations for Handoff support, some MacRumors forum members suggested that adding an inexpensive third-party Bluetooth LE adapter to an older Mac might be enough to add support for the feature. Unfortunately, several forum members have discovered that this does not appear to be the case.
One forum member has shared details of their testing, noting that in tests with a 2010 MacBook Air that does not include built-in support for Bluetooth LE, adding IOGEAR's Bluetooth 4.0 USB Micro Adapter works out of the box to add Bluetooth LE support to the machine, but does not enable support for Continuity features such as Handoff or AirDrop.
While OS X Yosemite remains in beta testing and things could change before final release, it appears that Bluetooth LE support alone is not sufficient to enable Handoff and that Apple is instead enforcing a stricter requirement for built-in Bluetooth LE support, perhaps by defining specific models that are able to use the feature.
Beyond Handoff, Apple is also adding a number of other features under the Continuity umbrella, not all of which require Bluetooth LE support. For example, the new phone relay feature that will allow users to make and receive iPhone calls right from their Macs relies on Wi-Fi rather than Bluetooth, meaning that more devices should be compatible with this feature and it is likely to work over longer distances.
OS X Yosemite and iOS 8 are scheduled to launch to the public in the fall of this year, likely around the September-October timeframe.