I'd like to quickly revisit the story because I've been following Pownce from day one, I've actually used it more than Twitter, and I've been bullish on the service because I thought it offered a compelling feature set when compared to other services in the space. I was wrong.
First of all, less is sometimes more. We all know that, but it was really obvious in this case: Pownce was the Twitter fan's dream come true, adding groups, file sharing and several other highly requested features without cluttering the service with too many options. However, sometimes simplicity and lack of options are what differentiates a service and makes it worth your while. Twitter devs may be slow when it comes to adding new features, but they're slow on purpose, because they understand that Twitter is good, for the most part, as it is.
Secondly, sometimes even being much more reliable than your competitors isn't enough for success. When you think of it, Pownce was positioned very well: it had the features, it had solid backing (it is, after all, Kevin Rose's baby), and - although it wasn't without technical issues itself - it came to the scene at a time when Twitter struggled with frequent downtime. And despite the fact that Twitter's problems with downtime lasted for months, users did not move over to other services, except perhaps FriendFeed (to a certain extent).