Williams co-founded Twitter and previously co-founded Blogger, the first really successful blogging service. He talked about how Twitter succeeded by taking away capabilities."Twitter, in case you don't know, is a very simple application built around the question: What are you doing?"

Constraints: how adding constraints can help your product. As developers, we want to add features. At the beginning we need to ask what can we take away to make something new.

Twitter is a blogging app, without tagging, formatting, templates, and posts limited to 140 characters each. He spent six years on Blogger, and most of that time was adding something new.

"The way that came about isn't by, we didn't sit down and say we want to build a blogging tool."

Jack Dorsey, CEO of Twitter, has the idea for a ubiquitous friends status network. And decided SMS is a great transport for that. Didn't want to write a new app for mobile. Drawbacks: Aside from messages limited to 160 characters, it's essentially a command line. "Imagine if your whole Web site had to be accessed through a command line." You'd have limitations. You can send messages, receive, subscribe, etc., all using SMS. And many do. But not most -- most use the Web site and third-party apps.

But because SMS was the primary platform, it made some difference. The interface is one field and a button. That gives it a "low cognitive load." Has a huge impact.

In retrospect: When added titles for blog posts, he hesitates more when he writes the post. "This may be a good thing, but it makes a difference."

Other difference: API. They've had a successful API from the beginning. Hundreds of applications have been built on top of or integrated into the API.

Interfaces for Skype, Second Life, emacs, and Outlook.

What if you wanted to do a photosharing service, compete with Flickr, limit to one photo a day and have no tags? That's what Photolog did, sold for $11 million (?), and had huge growth. It made photos high-quality, and people got lots of comments on photos.

YouTube has a 10-minute limit on movies. Because everything needed to be in short form, it was Web-consumable.

Podcasts, OTOH, could probably benefit from some limits. When running Odeo he thought everything on the site should be 10 minutes or less.

How about a social network that limited you to 10 friends? Don't have to deal with lots of friend requests.

How about e-mail with 20 messages?

"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."

"What if you wanted to create a competitor to Yahoo, but went totally minimalist? You had whitespace, and a button."