Of course Twitter is status updates. The very thing it asks of you when you use it is, "What are you doing?" Naturally, most people just answer that question.
Twitter, however, is what you make it. It is status updates. It's also searching (Twitter Search), photo sharing, mashups (a combination of Twitter data with information from another Web site such as a news organization), open API projects (TwitPic, for photo sharing, was one of the first), hashtags (a tag appended to a tweet that makes it searchable by that tag) and a marketing tool for celebrities, politicians and businesses.