Android P
Alexander 🇷🇴:
Inline photos and smart replies: remember Reply? Now you have that built into Android. Developers can also add support for showing things like images and stickers in-line as well.
Screenshot editing: you can crop and draw on screenshots you capture. This has been a long time coming.
Volume buttons control media volume by default: Android does a lot of things really well, but audio management probably isn't one of those things. Your hardware volume keys control different things, without much of an indicator until you actually press one. Now by default, it's set to change media volume, as opposed to the ringer (as in Android O and before).
Simultaneous connection of up to 5 Bluetooth audio devices: the limit in 8.1 was two, one for calls and one for media. But with P you can allow up to five devices to be connected at one time. No simultaneous playback, though.
Zoom lens for text selection: like iOS has had since forever, when you select text, on P you can get a nice magnified view of the area above your finger.
Temporary rotation lock: a bit difficult to explain, but when you have rotation locked in portrait, a new icon appears on the nav bar when you turn your phone sideways that will quickly toggle to landscape, and vice-versa.
Alarm quick settings toggle: there's a new Alarm tile/toggle in quick settings. It shows the next alarm, gives you a shortcut to all currently set alarms and allows you to easily set new ones.
App notification tracking: if you get an annoying notification, you can easily track down which app it came from. That should help cut down on notification spam (if you have problems with that).
Wired charging sounds: Google never fails to impress. They've managed to miniaturize the machine that goes ping into the Android P developer preview, triggering it every time you plug your phone into power. Ah, technology.
Chrome-like 'feature flags': you know how Chrome has a bunch of extra options called "flags" for enabling testing features or tweaking things? Now Android has the same thing, called "feature flags." Basically, all the unstable testing stuff with convenient toggles in one spot.
Pulling down the notification tray results in vibration: Pretty simple. When you pull it down, you get a bit of haptic feedback.
Offers to hide frequently dismissed notifications: If you keep dismissing the same notifications over and over, Android P will ask if you'd prefer to just have them be hidden.
General visual changes
Display cutouts are supported (i.e., notches): The ZenFone wasn't a fluke, looks like we can expect more phones in the style of the iPhone X, and Android will have software support for them.
Time is on the left side of the status bar: probably in an effort to accommodate notches, Google has decided to move the status bar clock to the left.
Pixel Launcher is shaded, new mic icon, separate tab for Android for Work apps: Google's constantly making little tweaks to the Pixel launcher. Android P brings a few small visual changes, like a new shaded background and mic icon for the search bar, plus a new app dichotomy in the form of two tabs, separating Android for Work apps from personal ones.
New colors and tweaks in Settings: it's bright, friendly, and colorful, à la old-school TouchWiz. You'll probably dig it.
Ambient Display shows battery percentage, centers notifications: notifications are centered in Ambient Display, and you can finally see battery level without turning on your phone.
Night Light toggle shows active time: don't know when you set Night Light to trigger? Now the toggle/tile in quick settings will say.
Quick settings scrolls vertically: pagination is the devil, so with P all your quick settings toggles and tiles can live on one scrollable page.
Volume slider repositioned to the side of the screen, Bluetooth devices listed: Like the power menu in Oreo, Google has moved the volume sliders to be near the buttons that control them, and Bluetooth device volumes are conveniently accessed from the new interface.
Dat Easter egg, tho: In nine states you can have a comfortable night in with the psychedelic new Android Easter egg.
New cell selection animation: tiny tweak, but the radiating cell select animation looks a bit different.
Power menu screenshot button: this has been in many a custom ROM over the years, and it's more convenient than the hardware key combo. Good to see it hit Android proper.
Battery saver no longer makes things orange: it wasn't the most attractive look, but it did catch your attention. The notification is still present, though (and probably enough of an indicator).
Unlock pattern fades as you swipe: as you enter your pattern, the trail behind your finger will fade away.
Hawt new transition animations: not the biggest changes, but the new animations for switching between apps and activities keep things fresh. (Also, I think they look good.)
Text selection tweaks: Google seems to have decided that the various all-caps text elements in Android were a bit much. Now the text selection dialog isn't constantly yelling (and the "Web search" button is out of the menu).
Force stop/app version hidden: no functionality has been changed or removed here, but in Android P the "Force stop" button and has been sequestered into a menu button, and app version is in an "advanced" section.
Adaptive brightness is animated: the brightness slider moves in real time to reflect display brightness when set to Adaptive.
Modifications to existing features
Quick settings toggles are no longer expandable: some items in quick settings, like the Wi-Fi shortcut, used to be able to expand into a simple selector for quick changes, like swapping networks. As of Android P DP1, that is no longer the case.
Do not disturb is one setting, not three: "Total silence," "Alarms only, and "Priority only" are all gone, there's just one Do not disturb mode. At least you can configure it to individually match any of the others.
Location accuracy is all or nothing, no "battery saving" mode: "battery saving" mode is gone, location can either be set to use all connectivity options or only GPS.
Battery saver has too many names, configurable percentage trigger: Battery saver has three names, including "Reduced power mode" and "Low Battery Mode." You can also set it to trigger at any level between 5 and 70%.
Notification-type vibration configuration: you can individually change or disable vibration for calls, notifications, and touch.
Simultaneous file transfer and charging, USB menu tweaks: USB-related stuff is consolidated into a single menu, and you can also simultaneously enable file transfer and charging.
Battery menu nerf: per-app metrics and the Android O-style battery graph are relegated to a developer option.
System UI Tuner is gone (for now?): maybe it'll come back in the future, but the System UI Tuner and its various tweaks aren't present in Android P.It's still there, but it is hidden.
Tag Wi-Fi networks as metered from network details: this option was previously hidden away in the "data saver" section, having it on network details in settings is a bit more intuitive.
Apps built for 4.1 and lower may not work: The Play Store already has its own restrictions in place, but devices running P won't allow apps targeting Android 4.1 and earlier, even for sideloaded apps.
Bluetooth icon disappears from status bar when not in use: Saves you space when Bluetooth devices aren't connected.
Picture-in-picture mode has a settings icon: Easily access the PiP settings for an app to enable or disable the feature.
Privacy tweaks
Apps can no longer access mic or camera in the background: probably not great that they could do that before, but for the privacy-conscious, now you can be sure they aren't.
Fingerprint dialog consistency, unencrypted HTTP block for some apps: Google has introduced a new API for fingerprint sign in, hoping to make things more visually consistent when apps ask for your digits' details. Android
P also blocks by default all cleartext traffic for apps that use Network Security Configuration, though it can be individually disabled per-domain if need be.
Lockdown option to disable fingerprint sensor and other less secure login methods: a new "lockdown" option in the power menu disables potentially involuntary methods of entry into your phone. When enabled, no one can use your voice, fingerprints, or location to get inside.
MAC address randomization: helps prevent tracking on public networks. Long overdue, even if it's just an experimental feature right now.
Under the hood/API/developer stuff
HDR VP9 and HEIF formats supported: more efficient formats for storing video and images are natively supported by Android P.
Multi-camera API, display-based flash, and other camera API additions/changes: can be used for simple things like portrait mode parallax/depth detection, AR, and other cool stuff.
Measure distance to Wi-Fi access points: new location system for more precise indoor positioning.
New autofill APIs for password managers: password managers are so hot right now, and Android P will have even better support for them.
ART improvements—apps start faster using less memory: ART was introduced in as a replacement for the old Dalvik in Android 5.0 Lollipop, and in Android P it's better than ever.
Google <3's Kotlin: at I/O, Google revealed support for Kotlin, and now apps for Android written in the language will be faster.
More neural network/machine learning APIs: new hardware acceleration for neural network operations, and an expansion in API functionality.