On the plus side this could be really helpful for companies to converse on specific problems with the public. Or, more sceptically, it allows them to deal privately with problems, taking away the user’s power t publicly embarrass them when an issue arises. Of course the option to talk publicly is there too.
Another great move by Twitter to help braaaaaands and spammers. App.net was setup like this from day one, never had or heard of any abuse. I doubt that is going to be the case for Twitter users.