I would say fish is to zsh like what zsh is to bash.

More seriously, for a start: good defaults, highlighting and autosuggestion built-in, parameters search with help and completion... (but it's not Posix).

Regarding POSIX, I've been using Fish for about 4 years now.

POSIX always comes up, how it's a deal breaker.I want to mention that I build all my scripts as POSIX as I can or using Bash extensions. You keep having Bash/ZSH on your machine, so you can still use your scripts and don't miss anything. Shebang's keeps working,

    #!/bin/sh
    #!/bin/bash
    #!/bin/zsh

Personally, I actually don't change my default shell (chsh step). I simply set my terminal to use the fish command instead of invoking the default shell.

    - Gnome Terminal, there's a Title and Command tab. You can set a custom command there. Just put the path to fish
    - Terminal.app, Preferences > Profiles > Shell > Run command
    - iTerm.app, Preferences > Profiles > General > Command
    - Tmux, on your .tmux.conf `set -g default-shell /usr/local/bin/fish`

It's more portable for me that way.

I was also interested about it and from my research, Fish has two major differences:

- it enables the cool functionality out of the box, so unlike zsh you don't need to have large configuration file to enable everything

- it is not afraid to break bash compatibility to fix confusing scripting issues, so fish most likely will fail when executing a bash script, but writing scripts in fish should be more enjoyable