Records Activity
Being a data nerd I like information presented in 3s, it's just the right volume. Activity has this right and my watch tracks and informs on progress of exercise, move, stand. The interface on the iPhone is required to review, but without the watch it's little used.
Next Calendar Event + customised face
@Work I often have meetings 90% of the day and I need neither my phone, laptop, or diary. Just my watch. Especially useful to tell me where the next meeting is.
The customised face is also very useful. In the image I had the outside temp, date, and the time in Vietnam (offshore team) and England (Home). All useful at the time. Now I have Activity and quick links to exercise, relevant to me now.
Time travel (what's my next meeting)
Apple hid this feature in the recent OS, but for me it is great. Meeting running over? With a spin of the crown I can check what I have on next.
Ping my iPhone
Speaks for itself, I use this at least once a week. Anyone who says they would not use this is delusional.
Pay
I use this more often than all alternatives (card or cash) and more often than not the teller say "wow". I always respond with the same line; "soon everyone will have one".
Unlock my MacBook
I have waited for years for this feature. Facial recognition & fingerprint tech never really did it. The watch has saved me for entering my password 20 times a day.
Notifications
Handy, mainly because I actually read them on my watch. No matter where I am I can get the essence of the message and respond to the accurate ones.
Boarding passes
I don't fly enough to say I'd miss this feature. However, I dream of a day when all tickets are electronic and in my wristband.
Transport
It's an 8 minute walk to the station, do I need to speed up? A quick look at my watch tells me when my next train leaves. The app also detects where I am and so knows the station I'm heading too.
SMS
Receiving them is one thing, but the best feature is the translation from speech. It really does work and I use this all the time. Especially useful when driving.