If fish has a hook (har har), it would be autosuggestions. fish suggests the rest of the command as you type, like your web browser does for URLs. This feels faster than tab completions, because you can see what command you'll get ahead of time. It also lets you summon commands from history quickly, without having to think about it - just start typing.

If you're a die-hard bash user, autosuggestions are the feature most likely to pique your interest and make the command line more pleasant.