I think it’s also like walking into a chatroom with just the people you want. Any other chatroom (IRC, AOL, Yahoo Games!) has a bunch of folks that are ‘noise’ and you’re not always interested in. With Twitter you can tune them out (setting: only see @replies to people you mutually follow). This is important because my experience is tailored to me and different from everyone else’s. I follow different people so even though my social circles may overlap with hundreds of people I don’t have to be involuntarily involved in those conversations.
When my experience is just about me and the people I want to interact with then I find that valuable. Facebook, Plaxo and LinkedIn have screwed this up because they’re overwhelming me with information about people that don’t always matter to me. MySpace realized that we should be able to opt-in to our friends because, of many reasons, some people change their profile picture too much, or leave groups too much, or add applications too much, or… whatever reason you want.
Twitter means the updates I receive are from the people I want, the conversations are with the people I want, and even more importantly: the content is limited to what they want (to divulge). I believe this control is missing from sites like Facebook and is why so many people hide and remove all their feed activity. In some cases, those are the people I want to ‘follow’ the most.