One of the fundamental axioms of board games is that each game starts fresh, resetting back to an original starting point. Chess goes back to the original layout, as does checkers, Axis & Allies, Scrabble, Monopoly and pretty much every other board game out there.
Risk Legacy does not.
Instead, playing Risk Legacy causes the game board to permanently change so that the next game will be slightly different than the last game. The board itself evolves as games are played.
The designers accomplish this by using stickers that you attach to the map. Each sticker added essentially changes the map for all games from that point on.
Here's a very interesting thread with information from Rob Daviau, one of the designers of Risk Legacy. He outlines a lot of the reasons behind and aims of this attempt:
This led us to wondering why games always have to reset. Why are they a medium that always goes back to start? Movies and books are static forms of entertainment meant to be viewed but not altered. Games, by nature, demand that the user create the experience. We wanted to push that boundary to have lasting effects. Now you really create the experience. This game is not art to be hung on a wall but a leather jacket to be worn around until it has its own unique story.
I'm not sure how I feel about this board game. It is definitely an example of truly bold and ground-breaking game design. Will it be a success? Will it be a failure? I have no idea, but it is a glorious experiment.