Guild Wars Interview

PC Zone has an interview with Guild Wars art director Daniel Dociu (whose face, apparently, was used as a model for a HL2 character).
We have five full-time concept artists, and over half of our production staff are illustrators and concept artists. There's a large number of people who have diverse backgrounds, skills and interests who are encouraged to contribute to the creation process.

That's where the diversity comes from. As far as creature design goes, the wackiest ideas will be thrown around and our people will have the ability to illustrate them convincingly. Designers and writers post everything that they're making on a huge wall and whatever resonates with the team finds a home in the game. I think the important thing is to facilitate exposure for these ideas and see what clicks.