As per Sam Stoddard's article on the mothership, the nature of the "legend rule" and planeswalker uniqueness have been changed. While the rule still applies to your battlefield, you and your opponent may each have a legendary permanent that shares a name, and a planeswalker that shares a type.
This is the second time the "legend rule" has seen a significant change. The first was in 2004, when the legendary permanent already on the battlefield would not be put into the graveyard, making the first player to resolve a powerful legend even more advantaged.
This certainly will change the dynamics behind the Geist and Jace wars we've come to know and... appreciate.
Also of note, you are now allowed to register less than 15 cards in your sideboard, and are allowed to present any combination of 75 cards after sideboarding, as long as your main deck remains minimum 60 cards. From Matt Tabak's article: