I wonder how much it can do in the power reserve mode.
Time only seems like such an arbitrary limit. Time only still involves running the screen, which, if I had to guess, is by far the biggest power consumer within the Apple Watch.
As I recall, 80% of power on the iPhone is powered by the screen. After that, GPS is the next biggest consumer at 16%. Then cellular at 3%. Everything else is a rounding error (but of those rounding errors, Wifi is highest.)
So after running the screen, the next most power intensive tasks are related to using radios, and the longer distance the signal has to travel, the more power it requires. Since the Apple Watch only uses NFC and Bluetooth for radios, I don't think either of those will be a major power draw. I feel like the Apple Watch will devote 95% of power to the screen, ~2% to each of those radios, and everything else will be a rounding error.
Which means a power reserve mode, while making some slight sense on a phone (stretch the battery by 25% by turning off radios), it makes little sense on the Apple Watch (stretch the battery by 6% by turning off radios.)
Maybe my memory of how much different components consume on the iPhone is wrong and someone can correct me. I seem to recall it being in the Stanford iOS Programming course, when talking about how to make your app consume as little power as possible (it boiled down to not having frivolous radio communication.)