a pseudonym by any other name
At a very basic level, Google+ and Facebook are in the identity delivery business, and Twitter is in the information delivery business. That's a powerful distinction. It reflects a fundamentally different conception of what's more valuable: information or identity. It also gets at who is more valuable, advertisers or users. Twitter leaves room for identities to be fluid. Like Shakespeare's rose, Twitter cares less about your name than it does your words and actions. You may use a fake name, but you still smell pretty sweet.