This is by no means complete, but the features that I value would be:

• <Tab> cycles though completions as it should instead of duplicating the prompt.
• Completions also show help text (if there’s a provider for one). For example on git <Tab> it shows a short message describing what the command does. JJ goes further: jj diff -r <Tab> shows part of the commit message for the offered commits.
• There are just more completions than in any other shell I know. Aside from JJ the new Nix CLI also has great Fish completions and can dynamically complete flake outputs like package names.
• Entire commands can be history completed with <C-E> or <Right>. This completion is also directory-aware and can usually avoid suggesting commands with paths to files that don’t exist. In practice I find that it’s really good at suggesting the command I actually want to run, to the point that I rarely invoke FZF anymore.
• Abbreviations are in most cases better aliases since they do the same thing but don’t obscure what you’re actually running.
• No word splitting when expanding variables, because it’s never what you intended.
• Globs that fail to match anything are errors instead of silently doing the wrong thing.
• Control structures are a bit nicer (but that is subjective).

You can get most of these with liberal use of shell options, installing blesh, or alternatively installing zsh with a bunch of plugins, but Fish just has all of them out of the box. You don’t even need bash-completions.

how hard is it to transition?

It has a reputation of being very difficult from the past when it didn’t have &&/|| but I think today plenty of Linux users would not even notice. The most notable remaining differences are setting variables (requires the set builtin unless used to modify the environment for a single command), control structures (irrelevant in interactive use) and lack of !! (but you can make an abbreviation to bring it back).