Adam-- HomeKit isn’t Apple developing hardware that will be incompatible with everything else on the market. Quite the opposite – HomeKit is a communications protocol, and represents Apple’s attempt to bring some level of standardization to the top level network- and user-facing tiers of the fractured home automation landscape. This sentence from their introduction to the HomeKit documentation pretty much says it:
By promoting a common protocol for home automation devices and making a public API available for configuring and communicating with those devices, Home Kit makes possible a marketplace where the app a user controls their home with doesn’t have to be created by the vendor who made their home automation accessories, and where home automation accessories from multiple vendors can all be integrated into a single coherent whole without those vendors having to coordinate directly with each other.
An added bonus is that it will integrate nicely with Apple’s products, allowing you to use Siri to issue commands, for example.
My own feeling is that since SmartThings is such a major player now in the HA market, there will be an interface between ST and HomeKit that will allow the two to play together.