Two things. First, I disagree with the scenario laid out in the OP. I shouldn't have to tell my watch that I'm going to bed for my house to react appropriately. Otherwise what happens when you have guests who don't have Apple Watches? Or what if I get up in the middle of the night and want a few lights on for just 5 minutes or so? Instead, the home should be able to detect on its own, though various sensors, that you've "gone to bed." That probably means something along the lines of detecting that everyone is in their various bed rooms with doors closed, after a certain time at night, and no movement in the rest of the house for X number of minutes. These scenarios are conveniences, like unlocking the door when your car arrives home and the garage door opens. Having to ask Siri to do each one, every single day, would quickly get annoying. Notice that a lot of this can be done with other systems.
Ideally you should only have to use the Apple Watch for scenarios that you can't already do without one. These mostly involve situations where you aren't home. For example, tell Siri to adjust the thermostat while on your way home. Or get a notification when your kids get home after school, and, like in the demo, open the door if they forgot a key.
Two, we'll see whether any of this pans out or not. Home Kit has been out for a year now and I'm still underwhelmed by it. Other companies like Smart Things (now owned by Samsung) are a lot further along. I'm really hoping Apple TV comes along later this year with a lot to offer in this area.
