IGN sits down with Chris Sigaty for an interview regarding the mechanics and aspects relating to the Starcraft 2 campaign. In it, Sigaty states that "Starcraft 2 missions are all about player choice" and goes on to describe a very flexible and dynamic campaign, where players can choose "how much or how little of the game the player wants to experience." This lets people who aren't extremely interested in the storyline of Starcraft 2 enjoy the campaigns without being dragged down by details they might not care much about, and also gives people who are interested in the lore of Starcraft 2 a chance to experience every detail that Starcraft 2 has to offer.
Check out the entire interview, done by IGN:
Also Gametrailers managed to hit up Dustin Browder for a bit of his time in which he shares his thoughts on the direction of the campaign, and what they are trying to achieve.
Check it out:

Links
Starcraft Incgamers - Source
Gametrailers - Source