Looking at my widgets on iOS 13, I noticed only the Apple-created widgets (Weather, Shortcuts, etc.) actually conform to Dark Mode, and all third-party widgets still show light mode no matter what. I didn't realize this was something developers would have to enable individually, I kind of assumed iOS could just change the background of the widget automatically.
The most frustrating thing about iOS 13 so far was trying to find the list of Wi-Fi networks within Control Center. I tried for a LONG time trying to figure out how to long-press the Wi-Fi icon and Bluetooth icon to bring up their expanded menus, but no luck... until I read that you have to long-press in the middle of the widget, between the four icons, to expand the options. Nobody, and I mean nobody, would ever think of doing that without reading it first, so I'm hoping that's a glitch that will be fixed soon. Why on earth would they not bring up that expanded option list when long-pressing on one of the icons?
Overall very happy with this beta, though. Surprisingly stable!