Any user can immediately notice some of the 'chips' of the new OS (not only their architecture, but also the implementation is part of the OS). But the other part must first be implemented by the developers in the application software, and only after that the user will be able to evaluate them. Let's see what Android 8 Oreo offers.
Picture-in-picture
New multi-window mode (in addition to split screen mode into two windows). Application developers are required to implement mode support: indicating that the mode is working and setting parameters (for example, aspect ratio). Entering the mode is initiated by the user.
Notifications
There are some interesting for developers (but not obvious for ordinary users) possibilities related to hiding notifications.
First, you can find out that the user has swiped the notification. Thus, you can conduct detailed analytics on notifications (previously it was possible to record only the display of a notification and its opening by the user). And in the future, do not bother the user with notifications that he does not open.
Secondly, there is a timeout for notifications. It is convenient if notifications inform you about some time-limited events (the 'only today' promotion, for example). Theoretically, it was possible earlier to make a mechanism that tracks time and removes 'old' notifications, but now you don't need to reinvent your wheel.
Autocomplete
Oddly enough, but autofilling logins, bank card numbers (and passwords?) Works in existing applications out of the box. You just need to enable this mechanism in the settings. The developer is offered just a few additional features.
Downloadable fonts
For a typical business application, the thing is pretty useless. The fonts do not take up much space to save money on including them in the application. Much more expensive is the annoyance of the user that he cannot use the application immediately, but must first download the fonts. Another thing is some design applications and word processors (which make it possible to use a lot of fonts). Another option is a collection of applications that use common fonts.
Automatic font size
For small inscriptions (titles, indicators), you can set the automatic font size. There is, of course, fine tuning in the form of specifying the minimum, maximum and step (or even a set of specific values). A handy thing for multi-window mode (and split screens, and PiP). And if you also remember that the screens themselves are different in size and the fonts can be customized non-standard (smaller or larger), then the usefulness becomes even more obvious. However, this cannot be called a killer feature. Where it was vital, and before there was a selection of the font height. Only it was made 'by hand'.
Responsive icons
Now a simple icon can contain two border options (regular and rounded). In addition, the concept of responsive icons is introduced: in this case, a background image (background) and a foreground (foreground) are set separately. It is responsive icons that support visual effects.
Customizable shortcuts
Applications will now be able to prompt the user to add shortcuts for individual entities (objects) to the desktop. For example, you can add a contact from a messenger or an object from cards.
Maximum aspect ratio
Now the application can limit the supported aspect ratio of the screen (set for the width, i.e. a number greater than 1). The rest of the screen will be unused when the application starts. For example, you can set 1.78 (16: 9) and the application will not be able to use the entire screen of the Galaxy S8 or LG G6. But why this is necessary in practice is not very clear. After all, even without such a 'chip' no one bothers the developer to paint part of the screen black and not display the image there.
Moreover, this feature is ignored when using a universal stretchable interface (through it you can support multi-window, including picture-in-picture).
Multiple display support
When multi-window support is enabled, multiple display support is automatically enabled. An application can specify which display to launch a particular screen, but the user can move screens between displays.
Limitations of background processes
Now applications that are in the background and do not have any parts displayed on the screen (for example, notifications) receive serious restrictions on arbitrary work. Previously, this was one of the key differences Android from iOS: you could do anything in the background for as long as you like. But miracles do not happen, and such opportunities definitely affect the battery discharge. Now the nuts are gradually being tightened. However, such things should definitely be tried in practice and only then draw conclusions.
… and much more
In addition to the above, there are many other changes that a developer needs to know (for example, cache size quotas). See the official documentation for details.