Window management, mouse support, key bindings (I really dislike the defaults for a few of them), selection/copy/paste, macros etc
All stuff that doesn’t need to be configured but if you look at any tmux tutorial you’ll see boat loads of details about. Which can be confusing because how do you know if you don’t need to know it?
Also what’s the basics for one person isn’t going to be the basics for another. Eg I rarely use the detach capabilities these days whereas the panes and windows are something I do use lots.
My point is this: tmux is sufficiently complicated that a complete beginner might not know what features they need to learn.
That's a key feature when you don't reflexively use less and ] isn't a natural keybind for "let me scroll please".