Other people indeed chimed in on your question in other comments. But here's my take about the most important fish features:

* Instant auto suggestions, you just type stuff, like in a web browser.

* Sane defaults, no need to bring around your own config. On a new machine I just install fish, run fish_config and customize the prompt with the web UI.

* Scripting with simple syntax, especially for one-liners. Yes, fish is incompatible with bash, but it's less of issue in this case.