Charlie discusses the future of Twitter and touches on what I think are two key points: corporate twitter and content subscription.
The key as Charlie discusses is the opt-in and one-way nature of Twitter. That is, I only get updates from someone if I explicitly choose to receive them, and the party I subscribe to doesn't necessarily need to listen to me.
That sure sounds a lot like an RSS aggregator, doesn't it? To me, Twitter is exactly that: a messaging aggregator. The future of Twitter is a messaging platform.
Twitter has a number of ways to deliver updates - you can get them on your phone (via SMS), from IM, or on the web. And of course, you can get them as RSS and bring them anywhere you want. You can also send the updates from any of those mediums. Ever better, Twitter has an API for putting data in and getting messages out, which means I can update Twitter and have this "status update" sent out to Facebook, my blog, and so on.
Delivery based on context and priority
The key feature that would have to be added would be choosing how and when I want updates on an individual channel level. I may want to get updates from Charlie or Corey immediately because I'm more likely to meet up with them for dinner, but there are others I follow where I don't need to be notified as immediately (e.g., they don't live near me, but I'm interested in what they have to say). Today, I only have the option to turn on SMS notifications wholesale, not for individual channels. If Twitter is to become a messaging platform, this needs to change.
I'm envisioning something like a priority scale - I assign priorities to certain people and to certain devices. I always have my phone, so SMS is a surefire way to reach me. It's also the most expensive way to reach me (both on my end and on Twitter's end), so I don't want every message to get me there. Accordingly, the bar for sending me messages on my phone is going to be relatively high (and a big reason I haven't activated Twitter on my phone yet). Conversely, delivering something to my feed reader at home costs nothing, but is also less likely to reach me.
In the Enterprise
As Charlie discusses, the same problem exists in the enterprise. We have a number of ways to communicate with colleagues right now - e-mail, voicemail, IM, Blackberry push messages, personal phone and/or SMS in emergencies, and even post-it notes. Worse yet, regulated industries like investment banking and law firms may have retention requirements on all of those messages. Among other benefits, Twitter can offer retention out of the box and the ability to add new delivery mechanisms down the road with little risk.
We also build and maintain a lot of infrastructure and applications to deliver contextual information to our users. We deliver messages to the Blackberry, to desktop applications, to an Exchange mailbox, and to use within other internal applications. This infrastructure is effectively Twitter, though unfortunately is not nearly as centralized (yet - there's an effort to do that).
Imagine if we could simply have our users subscribe to different channels and then choose where to receive those messages. A critical issue (a server's down, or a stock price dropped more than 20%) might get delivered immediately to a phone, while something more mundane gets delivered to a feed reader. Obviously, this doesn't sound that much different to Newsgator on the consumption side (Newsgator has tools that cover most of the others and SMS and IM can be added), but the big advantage I see with Twitter is the simplicity in publishing. It's nearly trivial to send an e-mail - I mean, we can even do that in a SQL Server trigger.
Being aware of location
Another opportunity would be with location-aware clients. Imagine if you can subscribe to a channel of things close to me, which is especially powerful if you have a Twitter client on a GPS-enabled Blackberry, etc. When I'm in Union Square, a message from someone else in Union Square looking to grab a drink immediately becomes more relevant to me (perhaps more so than a message from Charlie when he's on his cross-country trip). Location awareness is even more important when you get into commercial content that Charlie discussed.
Of course, like all location-based services, the real challenge is understanding geography. When I'm in the West Village, I may be more likely to meet up with someone up near Lincoln Center than someone in Jersey City. This can be addressed, and worst case scenario being able to follow anyone within 2 miles of me is still a useful feature.
By the way, obviously I use Twitter. Feel free to follow me and/or add me as a friend, especially if you're in Manhattan or Brooklyn.