Whether it's 'hydras' or something similar, the single thing that got me into properly using Emacs was this feature in Spacemacs. For almost any situation, I can either pull up the spacebar 'global' menu, or a context-specific quick-menu. Discoverability has been insanely good, and I keep finding new tricks just by exploring the various menus that pop up.

I wish 'UI autocomplete' like this was a standard feature of operating systems. GUI's aside, how nice would it be to run a CLI command with some flag that allows you to explore the various options and string them together, rather than reading the manpage and searching for whatever flag you might want to use?