[ufo]

Fish breaks compatibility with traditional shell languages. The syntax is a bit different, the variable expansion rules are a bit different, etc. It also gets rid of a bunch of features and configuration options, such history expansion ("$!!").

The positive side of this is that fish is very simple and has a very pleasant experience out of the box. And this is not just about having "good defaults" - its also about having a good design that encourages having one way to do things. That said, an interactive shell is a very personal thing. The only way to really get a good feeling is to install and check it out.