Zsh have a long list of nice features that Bash lacks. Two of my favourites:
1. Imagine you are in a directory with file1.ext and file2.ext. You type f[TAB], it autocompletes it to file. So far, Bash and Zsh are the same. If you continue pressing [TAB], bash prints a list of files so you can complete typing the name yourself. Zsh starts cycling through filenames.
2. In Zsh, you can type for example /u/lo/b[TAB] and it automatically expands the path to /usr/local/bin. If multiple paths match the original expression, it shows a list and pressing tab cycles through the options.
Zsh is quite feature-rich, and for the most part a drop-in replacement for Bash. See this slide deck for more Zsh awesomeness:
