The built-in web browser is a cute feature, but all the prompts waste space with <username>@<hostname>.

Autocompletion - a feature I generally don't use in other contexts - and reduced functionality seem to be its main USPs. With readline, process substitution and subshells (for ad-hoc fork/join jobs started from the command line) it might be competitive with other shells.

For example, a reasonably common pattern for me, on the command line, is:

    $ for d in *; do 
    test -d "$d" || continue
    (
    cd "$d"
    time-consuming-task-1
    time-consuming-task-2
    ) &
    done

I don't think I could do this in fish today without writing a script, as it doesn't have subshells and can't run functions in the background.

There's also a big advantage in having the shell be the same (or highly similar) to your preferred ad-hoc scripting language. It means you can retrieve complex commands from your history and paste them into the script, with only a slight cleanup to parameterize them. A whole complex operation can be spiked and tested interactively before being put together into something reusable - possibly without foreknowledge that the task is going to be that tedious, or needs to be encapsulated. Zsh looks like a better alternative to bash for these reasons.