I love that it's more discoverable, with the keybindings displayed more readily.
I think one feature I use from Tmux frequently enough is switching between sessions. It's not currently something the zellij client can do.
Discoverability is #1 on my list of any new app. I don't want to spend weeks learning commands or referring to the manual to use functionality that I know is there. Over time, recurring patterns I'll memorize the shortcuts for. The rarer stuff I can go through the menu, explore my options and get tentative result using my own data.
You could also use a thing like a neovim distribution that has which-key support.
which-key (from emacs) is excellent, and I wish more programs adopted the idiom. I'd say in Helix's case, it's good for helping discover the long tail of features you don't use often. It'd be a struggle to use without learning some set of navigation/editing.
Without having used it, micro https://github.com/zyedidia/micro seems like it's a nano alternative to take a look at, too.
Do you miss it in Tmux? Not that I remember all the hotkeys, but most 10 of them used 99% of the time for sure, for the rest just `<leader>+?`, `Ctrl+a ?` for my config - shows your current settings.
I still think it's a neat feature for keyboard-driven interfaces to have. -- e.g. I'd rather poke around lazygit than tig.