For example, I have a 6.6" phablet, which I often use with a single hand when I walk. It's comfortable to look at the top part of the screen. However, it's much more comfortable to use the bottom part of the screen. That's why I love every app which considers this "single-hand" scenario and places inputs to the bottom.
Also, what do you like the most? Like small features, well-thought decisions, etc.
The last discovery for me was putting my favorite VPN in notification panel shortcuts to turn it on in seconds from anywhere.