I agree with 37Signals and their views on Mission Statements. Don't bother with one. 99% of the time they are useless words on a page and nothing more.

WordPress is something different to different people.

To me WordPress isn't some grandiose world changing thing. It's simply a content management system (yes, it is a content management system) that I really enjoy working with.

To other people it is bigger than that. That's okay. That is the beauty of the community. It's means different things to different people. There is nothing wrong with that.