Grammar: kakoune is heavily oriented towards selections. Things that happen in normal mode operate on the current selection. It's like using visual mode in Vim, except it's always on. It takes a little getting used to, since any cursor move changes the current selection. Part of its selection orientation is excellent support for multiple cursors.

Platforms: POSIX only. No support for odd platforms. If you want to use it on Windows, you have to use Cygwin.

Packaging: basically non-existant. You pull it from github and build it yourself.

Maturity: kakoune doesn't have version numbers as far as I can tell. I've had it crash on me, but it's rare.

Community: tiny but vibrant. There's #kakoune on freenode, which the core team seems to monitor pretty actively.

Syntax highlighting: yes, and it's more flexible than vim's. You can use it to roll your own colorcolumn, for instance.

Code folding and line wrap: these aren't there in kak, and I do miss them, but not enough to go back to vim

Tmux integration: awesome in kak, absent in vim. One of my new favorite things. In general, you can have multiple editors attached to the same session.

Automation / configuration: There's no such thing as kakscript, which I see as a plus. Nevertheless, you can do a lot of stuff here; I haven't scratched the surface, but there are lots of interesting example kakrc files out there.