Many websites which provide an API require developers to register and request a key, while others like Twitter and FriendFeed are completely open. Flickery, a new desktop client for Flickr, demonstrats why restricted APIs are bad. Flickr decided to revoke the developer’s API key because it was causing too much traffic, causing the Flickery application to stop working.
Requiring developers to sign up for a key before they start developing applications for a website will stifle development and make it less likely that we’ll see new and innovative applications. Twitter has maybe hundreds of third party applications thanks to their open API, while Pownce, which requires a key (even though it’s quick & free to sign up), has very few.