I don't think they had any patents protecting them, etc.
It's less about the service and more about who's on the service.
On April 17, 2009, at 10:11 AM, Oprah Winfrey tweeted: "HI TWITTERS . THANK YOU FOR A WARM WELCOME. FEELING REALLY 21st CENTURY ."
And that was it.
Twitter servers started crashing every fifteen minutes. A piece of artwork known as the Fail Whale was a constant fixture preventing the rest of us from carrying on our early adopter conversations.
Then, other celebrities started taking it seriously.
Then, journalists started showing up.
There are a lot of other services that work like Twitter. Some of them are even free and open source. Some, like App.net, have paid membership levels.
They just don't have the same people on it as Twitter, and that makes all the difference.