Finally got my watch on Tuesday, s4Al44 -> s7Al45, and I agree, the best feature by far is the battery & fast charging. The improved display is great, but isn't terribly noticeable unless using apps or entering a passcode. I use the Infograph black watch face, so the size increase isn't terribly noticeable outside of apps.
The extra sensors are nice too. I'm looking forward to looking at O2 trends over time, and it's nice to have a compass! AOD I could give or take. I'm leaving it on for now, but it's not an exciting feature for me. Raise to wake was perfectly acceptable.
I like having the option of a QWERTY keyboard, but even with a 45mm display, it's difficult to type accurately. I think scribble gives me faster, better results for most messages. YMMV
The s7 also seems to be optimized for WatchOS 8, whereas the s4 was not. I hesitate to say "planned obsolescence," but my s4 at 3 years old is at 97% on its battery health. The s7 has a better battery (~5.8% capacity increase from the s4). But the s7 is performing significantly better battery-wise on Watch OS 8.
My charging habit is 30 minutes in the morning & 60 minutes in the evening
s4 on previous WatchOS versions:
10-15% battery usage overnight (85-90% remaining)
45-50% battery usage through the day (50-55% remaining)
S4 on Watch OS 8 (even after unpairing & repairing at one point):
15-20% battery usage overnight (80-85% remaining)
80-85% battery usage through the day (15-20% remaining)
S7 on Watch OS 8 (paired using backup of s4):
5-10% battery usage overnight (90-95% remaining)
40-45% battery usage through the day (55-60% remaining)
Since upgrading to WatchOS 8, my s4 was finishing the day with only 15-20% charge remaining. I was having to leave it on the charger much longer than the usual hour in the evenings to get back to 100%.
With the s7 I'm back to the battery life (plus a little extra) I was used to when my s4 was on previous Watch OS versions. And that's with AOD turned on. The fast charging is really nice. It'll significantly shorten how much time my watch spends on the charger, which is great.
All-in-all, it was a really solid upgrade going from s4 to s7 and I'm really happy with it.