If I were Jack Dorsey, taking a fresh look at Twitter, from the point of view of Twitter Corp, here's what I'd look at.
What's going on in high school basketball in Berkeley, or how's the recovery effort with the nuclear power plants going. That's not something Twitter really is set up for. It's good for what's happening right now this second. If Elizabeth Taylor dies, signing on to Twitter will get me that information for at least a few hours after the event. But not if David Broder dies, for example (sadly, they both died recently).
News is and always has been what Twitter is about. There are other tools that are better for conversation and networking. The 140-character limit, imho, makes it useless for both. What it is good for is news.
Twitter is a new channel for sensational journalism. Maybe that's all they want to be, if so, nothing to do, mission accomplished. However if it is to become the news system of the future, as I've said it could -- many times -- then a bunch of corner-turns will be necessary. This is the first one. Giving it a memory beyond the last few minutes.