The $200 Fitbit Versa 2, arguably the Apple Watch's biggest rival, also has a display that stays on even when it's inactive. Fitbit's functionality, however, seems very limited compared to Apple's.
All of Apple's watch faces are optimized to work in always-on display mode, which slows the refresh rate down to as low as 1Hz to preserve power. That means you can choose which bits of information you want to see at a glance — like the weather, your next calendar event, or a recent timer — by adding them as complications to your watch face.
It also makes the transition feel more seamless when moving between the low-power always-on display and the regular display. Most of the watch faces I've tried so far on the Apple Watch subtly become brighter and crisper as you raise your wrist to activate the display. For certain clock faces that have metrics that refresh quickly, like the seconds on the Activity Digital watch face, those elements disappear when in always-on mode, and resume when the display is activated.
Even after only using the watch for a couple of hours, I'm noticing how convenient it is to see the time and weather just by looking down at my wrist without having to make any movements or touch the screen.
That's very different from the experience of using the Versa 2's always-on display, which only shows the time, battery life, date, and exercise statistics. It also doesn't transition into its regular display mode automatically when you raise your wrist like this new Apple Watch does.
Take a look at the images below to see what some of Apple's watch faces look like in always-on display mode. You'll notice that they're a bit dimmer than usual.