I'm trying to create a similar thread to
this one, because I feel the new UI is pretty usable now. Let me know if you find something else other than these and I'll add it to the list. Also I'll do my best to keep up with the new releases

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1. When looking at a contact info, press and hold a phone number or email address to copy it to clipboard.
2. You can paste a phone number from clipboard to the dialer by tapping the number display area (blue).
3. Slide down the top left corner of the titlebar to switch between running applications (also phone, sms and desktop) - sort of like alt+tab in Windows
4. Double-tap on text almost everywhere to bring up the copy/paste menu.
5. Double clicking the M button will start the music player panel in any application.
6. Slide the album cover to the left or right to switch between lyrics, spectrum display.
7. Pull down the middle of the titlebar to open task manager, or press and hold M button for 1 second / to close it either tap on the middle of the taskbar or press and hold M button again for 1 second.
8. Rotate the phone clockwise or anti-clockwise in Photo Viewer to switch view mode to big thumbnails.
9. To edit a group in phonebook press its icon (also possible to send SMS or Email to all of them this way).
10. Press and hold a message (not a group of messages) to bring up a menu (copy its contents, forward, delete...).
11. Slide your finger to the right to delete a group of texts (SMS) - also works in Notepaper, Ebooks, Call list etc.
12. Press and hold an image or link in Opera to bring up a menu (Open, Open in new window, Copy link).
13. Rotate phone clockwise or opposite direction in Calculator to use scientific calculator.
14. Press and hold the door icon (between phone and sms icon on desktop) to add, move or delete icons and widgets.
15. Slide the album cover image left or right to switch between lyrics, spectrum view and song rating in Music Player.
16. Press the volume keys to adjust digital zoom in camera (not working with M8SE).
17. Tap on the right side of the screen in Ebook application to open a scrollbar.
18. Double-tap on the spectrum display in Music Widget to switch to Album Art picture.
19. You can adjust sliders like backlight precisely by 1% if you tap on the left or right side of the line. Works in Music Player for volume and track position too.
20. You can hang up calls with the power button (press once). When there's an incoming call the power button will first mute the phone and by pressing again it refuses the call.
21. Possible to protect folders with password: Start Documents, check the little box next to the directory name, press Options, and select Rename. Then you'll have two password fields (password, repeat password).