What is the difference between Shuttle Adventurer (10213) and Shuttle Expedition (10231)
The LEGO Direct team's answer to this question is on brickset:


  Since its release in 2010, 10213 Shuttle Adventure
  has proven to be very popular!  Although the model was designed for a
  16+ target audience, our consumer call center has brought to our
  intention that many younger children have been building and playing
  with the model.  As a result, the Direct team has decided to look at
  the model once more to see what changes could be made in order to
  provide a better build and play experience for a slightly younger
  target audience.   10231 Shuttle Expedition may look similar to
  Shuttle Adventure in many ways, however its build is very different
  and offers several advantages.
  
  
  The fuel tank has been reinforced with supports to strengthen the assembly
  The landing gear is strengthened to function better across many different floor types
  The Cargo doors can now be more rigorously opened and closed
  The entire build has been reworked to limit opportunities for misplacing elements
  Astronauts now have wigs in addition to their helmets (1 male, 1 female).  Face prints are also different.
  The new set now has 1,230 elements
  New Satellite can be securely positioned on the new mechanical arm 10231 Shuttle