There will be a post commenting on the release later this week, but the main things are:

- neovim now follows the XDG spec: ~/.vimrc is now ~/.config/nvim/init.vim, etc.

- the viminfo files were replaced by a new system, "shada" (for shared data)

- lots of default settings have changed so it is more sensible[^1] by default

The rest is all the new features (when compared with vim):

- async execution (`jobstart()` et al),

- the extensions to the plugin system, so plugins can run truly asynchronously,

- the rpc api, that allows for extended clients.

I might be fogetting some things, but these are imho the most outstanding things.