Evan Williams, co-founder of Twitter and Blogger, believes that developers should be adding more constraints to applications, and fewer features. "Twitter, in case you didn't know, is a very simple application built around the question: What are you doing?" Williams said in a presentation at the Web 2.0 Summit this week.

Twitter has been called "micro-blogging," but it's really more like a big chatroom. Users converse by subscribing to each others' feeds of "tweets."

What's special about Twitter is how little it does: Unlike a real blogging service, there's no formatting, or categories, or support for HTML (not even hyperlinking, although URLs are clickable), or any other features we've come to associate with blogs. And messages are limited to 140 characters -- a legacy of Twitter's roots as a texting application, retained because it keeps Twitterers from running off at the keyboard.

The user interface for Twitter is a single text field and a single button, said Williams. That gives it a "low cognitive load" -- you don't have to think and plan as much before posting.

The more decisions you ask people to make, the more difficult it becomes for them to act. He said when he made the relatively simple change of adding subject lines to posts in Blogger, he started to hesitate before posting. (I've found this to be true myself, on my personal blog.)

Hearing Williams speak was a special treat for me: I'm a Twitter addict and was an early adopter of Blogger, the service Williams co-founded in 1999, and sold to Google almost four years later (although I've since moved on to Movable Type and then Typepad).

So, for me, hearing Williams speak was like going to a concert by a favorite musician.

I liveblogged Willaims's talk on Wednesday, but that was just a stream of realtime notes. In this post, I'm trying to actually connect thoughts together.

Williams, who co-founded Obvious Corp., Twitter's parent company, said the other key to Twitter's success -- in addition to its simplicity -- is a robust and public API, built in to the service from the outset, which allowed third-party developers to interface their applications to Twitter, leading to development of hundreds of applications for Twitter. These include the mundane, such as dedicated clients for the PC and Mac, and browser plug-ins; the practical, such as TwitterFeed, which flows any RSS feed into Twitter, and interfaces for Skype and Outlook; and the esoteric, such as interfaces for emacs and Second Life.

Williams concluded by noting that many companies have succeeded by applying the philosophy of added constraints. Fotolog is a photo-sharing site, like Flickr, but users are limited to posting one photo a day, with no tags. The service was recently acquired by Hi Media, a Paris-based interactive media company, for about $90 million. Because users could only upload one photo a day, they chose good photos to upload, Williams said.

Williams then he posed a couple of hypothetical examples of businesses that might succeed because of constraints: A social network that limited you to ten friends, which would prevent users from being overwhelmed by friend requests, or an e-mail application where you could only keep 20 messages at a time, to prevent e-mail overload.

Then he posed a couple of real-life examples as if they were hypothetical:

"What if you created a competitor to MySpace but you didn't let just anyone join. What if they had to be, y'know, in college. And you limited it to one particular college?" he said. "What if you wanted to create a competitor to Yahoo, but went totally minimalist? You had whitespace, and a button?"

In case you haven't been paying attention to the Internet this decade, Williams was referring to Facebook and Google.

What do you think? What's your favorite application that could benefit from added constraints?