My stab at the mission would be:
It is the mission of WordPress to develop tools that improve the community's ability to easily publish and manage online content, while inspiring the community to be kind, temperate, and diligent in every regard.

My thought when writing that were:
WordPress is about:

Making online publishing easier.
Making website content management easier.
Promoting Open Source Freedoms.
Promoting kindness and charity.
Giving the community the privilege to guide its development.
Constantly improving.

I used the word "kind" to refer to the GPL Freedoms. I used "temperate" to refer to community-controlled. I used "diligent" to refer to core, plugin, and theme developers, as well as other contributors.