1) A pretty great experience even before you start customizing. I only add a git status section to my status line and set some environment variables in my config.fish file.

2) An effort to clean up and orthogonalize shell syntax. The potential portability means I usually still make bash scripts when I'm sharing for others, but for my own stuff it's great.

3) Suggested completions. I start typing a command and if it's one I've used before in that directory it appears in faint gray letters on my command line and I just have to press ctrl-f to bring it up that way.

4) Probably a consequence of (1), but it feels very fast to me compared to when I used zsh. The author also talks about using threads and minimizing disk access to speed it up, but I don't really know how it compares to zsh on that level.