All future updates will be made to the 540 card cube.
Main Differences:
Each guild now has one well-rounded archetype, which may use multiple complimentary themes. This is instead of trying to support two diverging archetypes (i.e. Self-Mill & Deathtouch).
The percentage of multicolour cards is reduced to provide a smoother drafting experience.
The mana curve of the cube has been revised somewhat and the amount of removal present should also be better.
Of course, more cards allows for a more diverse pool. This should lead to a more interesting draft experience.