From what I've seen Twitter is great for powerful, dynamic people to read in tiny bursts and write in tiny bursts, then get on with what they were doing; the character limit makes Twitter the success it is. It also makes it fail at being a meaningful communication platform, given that concepts must be expressed in a finite, tiny space. Much Twitter anger seems to stem from a fundamental failure of one party to fully explain thier position within the character limit. But given celebrities' small attention spans and/or demands on thier time, I don't see them moving to another social network unless it makes the experience faster and easier to consume still. This makes Twitter-in my mind-less of a place for normal people to socialise, and more of a stream-of-consciousness of celebrities - a one-way social network, and occasional personal messaging client.
It's also great for bitching at big companies. They usually track twitter very well and I have even been fast tracked through customer service once because I tweeted that my telecom company was crap. The standard procedure if to get the complains off the web as fast as possible ;) It's also a great outlet when I'm simply frustrated. I have like 30 followers so I feel like I can post whatever my current corporate frustrations are.
That's Twitter in a nutshell - "for when you're too lazy to use PRNewswire."