With Q, Google is focusing in large part on privacy, giving Android owners finer control over what data they share and creating stricter limits on what information apps can ask for. It also includes small but useful changes to its interface and controls.
The Dark device theme from Android Pie seems to be gone in Q, but you can still turn on dark mode by tricking the system into using it through Battery Saver settings. Q applies dark mode across more of the system, including in settings.
Q gives you finer control over when an app can access your location. Allow all the time, Allow only when the app is in use, or Deny. The App info window also gets an Open button to go with the Uninstall/Disable and Force stop buttons.