I think one not necessarily has to do with the other. In this example Kevin simply used his existing network for the launch of Digg. I agree with you that Digg is a great product for the niche it is serving. That's why it is successful. Pownce on the other hand wasn't a good product and had limited success despite Kevin's efforts to push it.
I actually thought Pownce was an interesting product I'm actually kind of surprised it never took off, except it was too "Twitter-like" so it never got the network it needed to gain critical mass.
Pownce's failure had very little to do with it being "Twitter-like". It failed because it didn't solve a problem that real people had.
Launching a product that is similar to popular incumbents isn't a negative feature. In many cases, it can help your unique selling point. Mint: Quicken but online and great UI. Hacker News: Digg for startups. Digg: Slashdot by the people.
It gives them a way to quickly communicate feelings, and to see what other people are saying quickly. Because it's so short, you can follow a lot of news without being overwhelmed.
Async communication is key to twitter. It's also cool that you can tweet by any method anyone has ever thought of, from almost any device that can send any sort of data whatsoever. The API has helped them greatly. I'm sure that someone, somewhere has a computer that transcribes short voice messages that can be left by phone. If not they will by the time I hit "reply" ... there should be a rule #34 for twitter.
* e-mail
* sms (key for reaching mobile people w/o smartphones)
* web browsers
* native apps / widgets
And then services like twitpic built around twitter.
The problem of people sharing the inanity of their day to as many people as possible. And the problem of people who want to read about the inane things that people do every day.
