No I mean things like settings applets/notification tray/widget functionality being better integrated into the overall desktop environment.

And customizing just results in less "jank" than Gnome. IDK how else to describe it, but it just feels like a DE that was actually designed with functionality over looks. Not to say I think it looks bad, but there's definitely less emphasis on "looking nice" over having the most robust experience, which I appreciate.

I do also like the broad strokes similarities to Windows just because things are where I expect a little more, but not really what I meant by mature, if that makes any sense.

It looks like Windows by default.

Who cares whe you can make KDE look almost any DE you want? :p

There's no reason to stay with the out of the box look. Nearly everything is customizable and/or replaceable with an alternative.