I'm sure that if you take all the suggestions from this thread you'll end up with the mobile viewer having more features than the desktop experience!
Personally, I use the mobile to stay in touch with people, even on long boring flights. Communication is key.
1) IMs & group chat
2) Avatar to Avatar voice would be great
3) To be honest, for me, the graphics are totally secondary as I'd use my PC if I want to be immersed rather than turning my phone into a glowing hot brick. Also, being able to optionally disable graphics would be nice to save on data usage
From experience of using Lumiya and the likes, it would be nice to have a simple method of sitting on things close by without resorting to full graphics - just so you're not standing at home like a total lemon!
Enola's point about using HUDs is also essential for those times when you're not graphical but your wife tells you you have no legs............
In short, the beta testing and release can't come soon enough ;-)