I'm seeing a lot of comparisons between Apple Watch and smartphones, PCs, and even other "smart watches". I think that's missing the point of what Apple's creating here.
With the iPod, they singlehandedly redefined the way we interact with music (and later video). With the iPhone, they surprised and delighted people with a device that does exactly what you want it to, and more -- it was like a dream come true for me. With the iPad, they opened up an entirely new market for, as some called it at the launch, "just a big iPhone with no phone". With each of these devices, people tried to understand it (and dismiss it) by comparing it to existing technology and usage models... which led some to dismiss the iPhone because it had no physical keyboard, for example. Some pointed to the Nokia big-touchscreen phone to say "see, they tried it and it doesn't work". Until they actually used the iPhone and realized that, no, this is nothing like anything they used before.
My most anticipated apps for Apple Watch are the ones that allow communication without words. The ability to draw shapes to someone, and they can see you drawing... or tap, and it taps your friend... record your heartbeat and they can feel it... THAT's the killer app that has been missing from other "smart watches".
The apps that will do best are the ones that capitalize on the unique capabilities of a watch device, not ones that try to shoehorn existing models into a tiny screen. Glances, gestures, simple swipes and taps, voice (sometimes)... and context awareness to remove the first layer of interaction ("what do you want to do") and jump directly to the end result. For example, if I'm walking along the street and I glance at my watch, it shows me the time/date (of course) but also a simple map of the area and maybe landmarks/businesses around me. If I've never been there before, it checks my schedule to see why I'm there, or maybe goes into "tourist mode". If it's mealtime, hey, there's some restaurants nearby with food I like. If my heartrate is up, it automatically assumes that exercise is occurring and pops up a shortcut to related apps/data. If I'm moving at greater than 15 MPH, it assumes I'm in a car and switches modes to minimize distraction by default -- easily overridden by, say, a quick shake of the wrist to "wake up" the watch. SIRI, of course, but focused on verbal and graphic interaction, voicing an answer or giving directions (via taps even) instead of providing a page of links you have to interact with.
A device that closely monitors you, your movement, heart-rate, sound input, light sources, knows time of day and your location and your schedule and keeps track of who you talk to the most, what apps you use the most, when you use them... something that's always there at a glance when you need it but knows when to shut up... this is a much more personal device, more closely tied to every moment. IF IT'S DONE RIGHT, with careful and intelligent design, this is a game changer.
When they released the iPod, iPhone, iPad, each time, Apple changed the industry, our assumptions about what's possible, and the way we live. (Even the most skeptical have to admit this -- who here doesn't have a smartphone?) I have no doubt that we're witnessing this happening again.