The WordPress Philosophy page is hard to read, so I made this boiled-down version.
Great out-of-the-box - You can install WordPress in five minutes, and little extra configuration is required for a great experience.
Design for the majority - The userbase is mainly non-technical folks. Design the software for them.
Decisions, not options - The average user has no interest in tons of customizability, so don't offer it to them.
Clean, Lean, and Mean - WordPress provides a basic featureset, and integrating any new functionality into core should be used by 80% of the users.
Striving for Simplicity - Simplicity is good.
Deadlines are not arbitrary - One time a WP version took a year, that sucked.
The Vocal Minority - Engage users for feedabck proactively. Users that voice their opinions voluntarily are a minority of our real userbase.
Our Bill of Rights - see the GPL's four freedoms.
The project has taken on other guiding principles, which we should find a place for so the community can access and understand them better.
The "mission statement" of the WordPress project is to democratize web publishing which is mentioned on the WordPress foundation's site but not on wordpress.org.
Made for the lowest common denominator. An expression of Democratizing Publishing. WordPress should be easy to install on common shared web hosting. Many hosts may limit a user's access to configuration for Apache, PHP, etc., so WordPress doesn't require access here.
Support for Internationalization. An expression of Democratizing Publishing. Your language and location shouldn't affect your ability to use WordPress.
Backwards Compatibility. Things shouldn't break for the user. Sorta fits some other philosophical tenets, but not neatly into one. Inherited from b2's commitment to ease of use.