Material You. I wondered why they wasted resources for ... colors. But it's so nice to have a consistently colored UI across apps and across dark/light modes, and I wished that more apps would support it. Also, those pastel colors are less stressful for the eyes than the previous grey/blue.
I know it's not everyone's taste but I really like it.
I have to respectfully disagree here. I would like to be able to choose what that color is. I HATE when I use a picture of my orange cat for a background and all my apps are brown.
If there's some way to override it and choose your own color, I haven't found it.
Over on /r/Android there was a very vocal crowd that saw it not only as a gimmick but actively detested it. In their opinion an UI is only good when it has an AMOLED black background (and 0 px padding between UI elements, but that's a different topic).