It's important to note that with the shift in focus from one-on-one conversation, the technology of transferring the data changes as well. Now, sentences are queued up one at a time on the user end and then sent out when the user hits return. This seeming subtle difference really does make a difference when one is participating in these chats and really does change the nature of the conversations. With entire thoughts getting queued up before getting sent out, conversations lose a certain amount of immediacy.
By having to wait until a sentence is finished, responses to what is being said have to wait until that one person has completely finished thinking. Unlike in real conversations, where even when social convention expects us to wait until one person is done before responding, we can still begin to formulate our responses before-hand, while the other person is still speaking. But, by forcing us to wait on dead space while the other person is typing, the flow of information is greatly reduced.
Not only that, but when there are multiple people in the same discussion, the problems become even more apparent. All these people will be waiting on the words of one individual to finish typing. Once they get the words, they will all read the words, then formulate their own responses and send them. All of a sudden, the space is deluged with the responses of everyone else in the room, making it difficult to wade through everything being said.