I completely agree with Philip Michaels at Tom’s Guide: Photos in iOS 15 is a great upgrade in lots of little ways. The information card that reveals EXIF data is way better, and I appreciate the better Memories features.
Best of all, for me, is the Photos widget. I cannot remember what it was like in iOS 14, so it might not have changed that much. I added the iOS 15 one to a home screen stack when I installed the first beta and it makes me smile all the time. It somehow manages to surface photos I have not seen in ages but are absolute gems. Not always, but with more consistency than I expected from a robot.
That is the best tip I can leave you with: add the Photos widget somewhere on your home screen. Maybe it will light up your days as often as it has mine.