SMS to mobiles is a must.
[Following part edited by Moderator]
Horizontal screen layout when typing in chat. Please create a separate idea for that.
Send/Receive photos. Separate idea here
this is the critical missing piece!
Add SMS messaging to the WP app! I wonder why no-one thought of such a great feature before. Oh, wait, it's already on the bendPhone and lagDroid apps. No love for Windows Phone, MS?