Here in this presentation, Guy Kawasaki makes a very valid point about how to write mission statements. He gives some examples of how it should be...It should be 3 words instead of a paragraph to describe the mission statements.
In a different perspective, I found some mission statements with few words but it's totally vague...So, as Guy says here, it's very important that you write it clearly.... Otherwise the outside world may even think that you don't know what you are doing! It's a good tip for the startups.