Much of the more real-time nature is the approach it takes to pushing the posts you make, rather than waiting for people to pull them. Twitter can push direct to your IM account, your cell phone by SMS, or to whatever client you are using, many of which ping the user when new posts show. And the short length leads to more casual two-way conversation. Think of it as a little bit like a chatroom for you and your social network without any of the overhead.