I think the main difference is timing. In a talking conversation, there are fairly established norms when it comes to timing - you say what you want to say in a smooth and consistent fashion, then the other person immediately responds. If you falter or stutter or pause just a bit too long, you throw everything off and the other person thinks you're strange.
In a typing conversation, you can type at your own pace, taking as many pauses as you need, then you press the enter key and the other person is none the wiser. If you take a long time to respond, perhaps you were just away from the chat window for a bit.
The difference is even greater in things like forums, where the conversation isn't instantaneous at all. You can type what you want to say, edit it, revise it, rewrite it, do whatever you want, and then post it, and nobody has any idea what went through your mind, face, and fingers as you were typing it.
I think that's why the internet is so dangerously inviting and addictive to us social phobes - we can make ourselves come across however we like. No more stutter, no more stumble, no more awkward pause and uncomfortable shifting. On the internet, smoothness is proportional to literacy as opposed to social skills, with that dumb loudmouth who has too many friends coming across as an awkward loser.
I've always thought it would be a nice change to modern culture for the customs of conversation to include a short pause between statements so as to allow adequate time to come up with a decent response. Just imagine how much more substantial conversations would be. People might actually listen to eachother without feeling like they're just waiting for their turn to talk.
I can be myself with people.Where as i go to bits in real life and i worry if i ever met someone off the net would they think wow this person isnt who he made himself out to be.
