Differentiate windows by changing the taskbar icons
A feature that I think would be great is to change the Icon of the taskbar according to what the conversation is. (many other IM softwares do this thing, and it's great.)
If it's a group chat: to have a specific icon that shows that it's a groupchat If it's a "normal" conversation and the contact we are talking to is offline, or busy, or unavailable: change the icon according to his state.