Most shell scripts start with a reference to the program that should execute them at the top of the file (i.e. #!/usr/bin/bash), so this shouldn't affect most people, especially since this change will only apply to new users.

Depends on the default Zsh configuration file they provide. When I used Zsh in the past with just the default config I found some Bash 'features' missing. Stuff like reverse-i-search (ctrl-r) and tab-completion. Most of them are simple config changes but they don't make a default Zsh a drop-in replacement for Bash from a user perspective. I think this is also the reason projects like Oh-my-zsh.

No, because the syntax of zsh is mostly similar to bash. Yes, because it's only mostly similar. For scripts that start with `#!/bin/bash`, no difference at all, because they will still run with bash.

If you just type commands into a prompt, then no, not really. If you do anything more advanced than that I think you’ll notice the differences quite quickly (I did).

`brew install zsh` and find out.