I got mine as an unexpected gift, so I was mainly thinking about the notifications since that is what I often see in reviews.
The unexpected things are numerous, but I will go over a few below:
1. iDevice Control - I have a few iDevice products that plug into the wall and you plug lights or something into them. With the watch, I can now turn them off and on with Siri. Very convenient since I don't need to track down my phone to turn lights on outside.
2. iGrill2 - this is another iDevice product. It uses the iPhone via Bluetooth to monitor the temperature of the grill and the food on the grill with probes. One issue with Bluetooth is the range. However, with the watch, it effectively doubles the range since it connects to the phone which can be placed inside the Bluetooth range.
3. Controlling Apple Music and Airplay - This is one of my favorite features. For example, I can be in the kitchen with the iPhone docked in the bedroom and tell it to play the Moondance album and then use the interface on the watch to select the Airplay speaker in the Kitchen. Since I have Airplay stuff all over the house, it is very convenient and useful. (Note: It is nice for controlling podcasts as well, but I haven't run across one that has Airplay built in like the Apple Music app. To get around that issue, I launch the Apple Music app, select the Airplay device, then go back to the podcast app and it plays where I want it to play)
5. WeMo control - In addition to iDevices, I have a WeMo power plug product that is similar. While it can't be controlled with Siri, I have a "Do Button" setup on the phone (via Glances) that I can push to turn it on and off.
6. Numerous Remote features for media - It can control everything in my home theater (and most of the stuff in my office) with SimpleControl, Remote app, Directv app, etc. When I am laying on the couch and the iPhone is out of reach, it comes in handy for pausing a movie and restarting it.
There are plenty more, but those would be the top six I didn't anticipate.