Completely overhauled styles, including default and dark themes
Reply to, star and flag messages
Slide-out Sidebar for easy menu/star list access
Tweaks to make typing easier (including Return -> Send, @username completion)
The app works by injecting extra CSS and JS into the standard Stack Exchange chat window. It comprises only of client-side code. No data is sent to any servers other than those belonging to Stack Exchange.
For anyone interested in running this apps mobile chat interface on your desktop (as in a sidebar chat window with reply support!) the author has responded in record time to my request and provided a userscript version!