Essentially, the Master Edition is a unification of a bunch of the core worlds and settings in ICRPG. The design and layout was made to be consistent throughout the different sections, along the same lines as what you see in the Quickstart 2e.
Like you know how when you open up Blood and Snow or Vigilante City, they had their own sort of feel and design? Now they are all consistent throughout the book.
Additionally, things like MAGIC were revised a lot so that it doesn’t feel so much like its own separate system. Now, systems like Stun Powers from Vigilante City and the advanced MAGIC system feel similar. This makes it a lot easier to jump from setting to setting.
Looking through it, my own feelings are that Master Edition is really going to facilitate a GM’s mastery of all the ICRPG settings and worlds. That’s what has really changed between the original books and the Master Edition. You’re going to feel like you have a good grasp of everything ICRPG has to offer.
Hope that helps @ArtfulDodger