The most obvious new feature, which got a loud, welcoming response during the keynote, is Dark Mode. It's not just a night-friendly mode that saves your eyes; it saves your phone's energy, too. Go into Settings and search for "Dark" and select the Dark theme (as opposed to the Light alternative). This flips the color scheme from dark text on a white background to the reverse throughout the system and in supported apps.
It's important to note here: not all apps support the dark theme. In this beta, at least, supported apps are mostly Google's own, like Photos, Messages, YouTube and Dialer, but these already had a version of the dark interface. Calendar does a strange thing where its startup page will be black or white depending on the mode you're in, but the actual interface is the same light background in both cases.
Google said that it's developers who need to make their apps display a different color scheme when dark mode is activated, and it has tools to help them with that. One of them shows developers what their apps would look like dark mode simply by flipping its colors. If they like the way that looks, the tool can cut down many of the steps needed to add support for dark mode. Otherwise, there are other tools to help them figure it out.
For now, Dark Mode on the Android Q beta 3 is not yet truly system wide, but it's a nice start.
Don't expect the beta to be the same across all compatible Android devices, though. While the beta is available across 21 different phones, it's up to manufacturers to decide which parts of the UI they want to implement. On a Huawei Mate 20 Pro, for example, I tried to enable dark mode but saw no difference when it was on or off. I also couldn't find gesture navigation or dark mode on some of the other handsets on display, like an Essential PH-1 or a Nokia 8.1. Only the LG G8 I was using had all the new features. Basically, your experience with this beta may vary depending on the phone you have.
Another significant change is in navigation. With Android Q, you'll be able to use a fully gesture-based system that resembles the one in the iPhone X and XS. That means you can replace the triangle, circle and square (back, home and all apps) with a swipe-based system. To be clear, you could already do this in Android P, but Q introduces some new gestures and completely removes the back button altogether.
This isn't a mandatory or default mode (not yet, anyway). You can enable this in the settings by searching for "gestures." When enabled, you'll find a bar at the bottom that's slightly wider and thinner than the "pill" in Android Pie. While you could tap the pill to go home, in the Q beta, the bar doesn't do anything when pressed. It's more of an indicator for where you should be swiping.
Flick it up quickly to go to the home page and swipe sideways on it to switch between your open apps. Go back by swiping in from the right edge of the screen. You can pull up the app drawer from the home or recent apps pages, though not if you have an app open. You also no longer summon Google Assistant by long pressing on the pill or home button; instead, you have to drag diagonally up from the bottom left or right corners of the display. Or, you know, say "OK Google."
I was a little upset at losing the back button at first, but over time I realized I like swiping from the edge of the screen. Since I can do so from basically anywhere on the display's edge, so it's handy when I'm gripping my phone nearer the top or holding it horizontally. And if you don't like it, you can always go back to a more familiar button-based interface.