The simplicity is for sure Twitter's main success factor. The service itself from a technical perspective is really ridiculous and still people like it so much. One of the nicest things is for sure the integration with IM tools or your phone. So you're for instance able to directly "twitter" from your favorite instant messaging tool such as Google Talk or even from your phone. You can read my twitter entries directly from my blog here (see at the right) or at my Twitter page.
Go to Twitter's homepage if you want to read more.