GPS can be disabled, thus battery life is extended beyond.
Afaik GPS is automatically disabled if the coupled iPhone is within reach of the Watch. So people who always have their iPhone with them are most likely to see the improved battery life.
Why are so many people wishing for an always on display? Genuine question... I lift my arm and my display comes on, when I'm not looking at my watch it's off and not wasting energy. I genuinely don't see the point.
As has been written: The mechanism is not perfect yet. Neither in activating when desired nor in preventing "false alarms", costing energy and drawing unnecessary attention.
Also, sometimes you do not want to actively raise your arm to read the time or incoming notifications, as it could be disturbing in certain situations (even be considered rude) or simply because one is lazy or busy with his hands doing other tasks that prevent raising the arm.
And finally there is the psychological aspect: Even if the wrist raise mechanism is working, you still have to wait for some 2/10th of a second before the display is really on. While this is not directly relevant in most (if not nearly all) situations, it gives the
feeling of the Watch being somewhat laggy. And if Apple did not completely abandon user experience, this is something they have to consider.
My thoughts as well - My main hopes now is just for an awesome software launch this summer with WatchOS 4.0 - There are small, but very useful stuff they could do pretty easily without dragging down power
I'm really looking forward to how they will evolve watchOS. Chances are that the possible optimizations are pretty much already included in watchOS3. What's up next are new features and a general evolution of watchOS. However, I'm expecting those to be (in part) limited to Series 1 and above, as Series 0's hardware is already at its limits with watchOS3.
watchOS4 will most likely still run on Series 0, but owners will probably have to consider a tradeoff between speed and features before they upgrade.