The application requirements are similar to bbm (BlackBerry Messenger) / kik:

Web Page
- register
download / landing page

wp7, Android, iOS app:
- register
- login
- push notifications
- settings > save conversation history
- reads if message has been successfully delivered and also read
- adding friends > from facebook
> from twitter
> email / userid
> mobile barcode scanner
- very light weight app
- avatar / status update
- use of live tiles for wp7 with actual working push notifications
- smilies

PC / Mac:
- work flawlessly with the mobile versions

I think that is everything.