The auto-complete is just that good. It will autocomplete options for arbitrary programs intelligently. For example I use sshfs on a few different machines in my home and can simply type the name of the program and press the up arrow to get the list of every previous sshfs command string I've entered. It uses a history file like bash but can pick out only the things pertaining to the program I'm trying to run.

probably the biggest thing for me is that command substitution translates output into arguments by line so that things like

    rm (cat list-of-files)

just work without any quoting nightmare regardless of what kind of weird characters are in filenames (well, at least as long as they don't contain newlines). that just simplifies working on the commandline a lot.

search feels more convenient although the difference seems superficial and i can still see room for improvement (i'd like to be able to type two words and then search for commands with those two words in any position)

the syntax and other things like array handling feel a lot cleaner.

multi line commandline editing felt like a great idea too although the use was not very intuitive (i think they removed that feature?)