If you think back to when Marshmallow was unveiled, and then even further back to when it was still known as 'M', you may remember a feature in the System UI Tuner, known as 'Demo Mode.' It was a useful setting, especially for developers or others needing to take screenshots (ahem), since it replaced the notification bar with a generic, preset one which did not show any existing notifications or low battery warnings.
It disappeared in Developer Preview 2, though, seemingly gone without a trace. As with everything in the System UI Tuner, things are liable to disappear, break, or be removed at any time, so this wasn't entirely a surprise (see David's comment from a few days ago for more info).
The camera might be, overall, the most complained-about feature in stock Android. While other flagship devices such as the Samsung Galaxy S7, the iPhone 6S, and the HTC 10 have gratuitous features provided in their camera apps, the Nexus devices have none. No white balance adjustments, shutter speed corrections, or different filters. Maybe the most obvious option that has been missing is exposure settings, which just about every other device worth its salt has.
However, if dev preview 2 and 3 are of any indication, an option to make changes to exposure may be coming back. In Dev Preview 2, the Pixel C had a Manual Exposure toggle under 'Advanced' in Settings.
It would definitely seem like Google is making Android N the 'polish' release: things which haven't seen any changes for years, like the System UI icon, are getting refreshed, while features are not being included in the final release because of a lack of polish (like the dark theme and night mode). Even the update procedure is getting updated.
As well as the aforementioned System UI icon, another long standing Android fan-favorite is getting a revamp: the OS update animation. Gone is the little Android figure with his front open, a prism turning, and his antenna going. It feels like it's been in Android forever, but it might be gone soon: instead, the new animation is a swirling circle of light, which looks great but maybe has lost some of its inherent Android-y-ness.
Since 6.0.1 Marshmallow, Android has had a cool feature whereby you tap the power button twice in quick succession and the camera app will open. It's a handy feature, and one I use reasonably regularly to quickly snap a photo.
However, ever since it was introduced, there's been an issue: if the phone or tablet is unlocked and the power button is double tapped, the device will first lock and then open the camera app, rather than just opening the camera. This behaviour is definitely present in Marshmallow, and we're reasonably certain it was the same for N Developer Preview 2 (DP2), although we can't actually find a device to test on (if you have DP2 and find this isn't the case, let us know in the comments).
Quick settings is the site of major improvements in Android N—there's a new row of five toggles along the top of the notification shade, the quick settings can be rearranged, and you can even add new tiles. The latest Android N Dev Preview 3 brings more improvements and tweaks to quick settings.
The first two N Developer Previews were alpha releases, so naturally a good number of things didn't work correctly. One of the apps that purposely did not work as intended was Android Pay, which produced a screen saying it was disabled until a future release. As Developer Preview 3 is now officially a beta, the Android team has seemingly seen fit to restore Android Pay to working order.
The reason Android Pay now works is because Compatibility Test Suite (CTS) is now approved. This also means other apps that depend on CTS should work too. On Developer Preview 1 and 2, this was not approved, and so Android Pay did not work.
You know how you can usually type some combination of Alt or Ctrl or Cmd with the question mark on your computer to surface a list of available keyboard shortcuts in whatever app you're using? Android is about to get the same option, which is great if you plan on using something like the Pixel C for work.
The trigger for the overlay is Alt + / (on Chrome OS, it's Ctrl + Alt + /). Once you tap that on your physical keyboard, it should show a screen with all of the keyboard shortcuts that are available from the system and your currently used app.