That is really the key question, it's all about compelling use cases. But Tim already gave us a big hint at one of the D conferences.
First, if you look at the wrist, there are 3 benefits to a wristworn device:
1. Fashion - visible to all, it allows for a personality statement
2. Convenience - always visible to you, it allows for immediate viewing/access to information/controls/features
3. Body measurement - directly touching your body/skin, it allows to measure your body functions
Fashion - it has been done thousands of times before and though I am sure Apple can innovate here and bring its unique hardware+software touch (how about a band made of flexible display, allowing for custom skins instead of changing physical bands?), I think fashion alone does not justify a new Apple product category
Convenience - Warmer. Again, done many times before but still with room to innovate. Cue payments, personal identification and the Disney Magic band which Disney (connected with Apple through the board room just as Nike is!) already employs in their Orlando park (there is a great article about it in PC World). Again, it's starting to be intriguing but such product would still not fully make "a significant contribution" which would justify a new Apple product category - the rarest of breeds.
Body Measurement - Hot! Tim said he was most intrigued by the possibility of a wearable device to change one's behavior. If you think about it, our always connected devices tell us million different things from your stocks value, weather in Rio, current news from Asia. But none tell us anything about the most important and priceless thing in our life - our health!
I believe fitness, health and sleep monitoring will be the main drawing card. For the first time, a person can constantly monitor his/her health, feed the data to his/her physician, get feedback and/or learn to interpret the data with the physician's help and - adjust behavior. Also, there can be a groundbreaking interaction with apps (as already suggested by Apple patents). Let's say my calendar for tomorrow morning is clear. I set the alarm for 8 AM. But I fall asleep only at 1 AM and my sleep is interupted several times. The device then (if enabled) automatically changes the alarm to 9 AM as it recognizes based on past measurements you need at least 8 hours of sleep. And the list of possibilities could go on and on.
Conclusion - though we can count on Apple releasing a beatifully designed product with many convenience features (iOS 8 and recent Tim's comments strongly suggest quick contextual message replying and voice messaging), the least explored but potentially the most benefitial uses cases are the ones related to payments, personal ID and mainly health. In my opionin, measuring health and the body-app interaction can brake new grounds.
You talk of opionin—have you been taking opium and onions simultaneously?
An earnest post.
Meh. Health is niche. Payments may be gold.