Health Alert
It's an app for health warning. Since we don't really know much about the health sensors for Watch, this app could be anything (open for future additions) including but not limited to Cholesterol level, blood sugar level, heart rate, etc. based on the sensors within the Watch. Imagine how many life can this feature save if it's working as intended.
We do know that the only health sensor the watch has is a heart rate monitor. Some third party companies are working on tracking blood sugar levels, but that requires a separate sensor inserted under the skin,
Relationship
It's an app for communication through gestures to your loved ones, ie. you draw something of the watch face and the other person sense it in real time.
The watch lets you send your heartbeat to another person. The taptic engine can't let the other person feel a drawing -- it just taps your wrist. It's nowhere near tracing drawn patterns on your skin.
Self Improvements
It's an app to remind you gently (subtly: nudge nudge) at specific time to: move around, drink water, take a deep breath, etc.
Goal/Target of the Day
It's also a simple app to shows your progress during the day (in a clock face) of what you set as your goal of the day, be it writing that chapter of book/blog, 30 minutes running, steps, studying that particular subject, etc.
Apple announced that the watch has a function to remind you when to stand. It also has an app that tracks how much you move during the day, and shows how close you are to the goal. Other kinds of reminders and goal-keeping should be easy enough for third party apps. I expect to see a lot of those once the watch is released.