Group status, 1-on-1 status, mood - all that mammalian stuff. Most of that comes through side channels: posture, tone, eye movements. As for equivalent side-channels in online communications, compare:

• u can say alot of things wihout words rite?? :)
• I'm sure you've already thought of this, but have you considered the information content of syntax as well as semantics? Sorry to bother you.
• Augh, RTFM you n00b, don't waste our time with this BS. Punctuation, tone, and w0rd ch01c3 are covered on the wiki for online chat.

You can put a chimp in front of a keyboard, but it's still a chimp.

What I find fascinating are the different levels of subtext-literacy I encounter online. Everyone is hard-wired to internalize their culture's body language model at a young age, but exposure to the internet equivalent varies widely. I imagine that being internet-communication-illiterate is something like visiting a foreign country - you can understand and make yourself understood, but you stick out like a sore thumb and are blind to every kind of subtlety.