Rovio obvoiusly took the same aproach Nintendo did with the Mario franchise (Super Mario Galaxy, Super Mario Galaxy 2). Not to say that the game is bad or unoriginal. Infact, it's just as addicting (if not more) as the original Angry Birds.
The plot is the same damn thing it's always been, except for the addition of a boss battle at the end of each world, which is incredibly easy.
A new bird was also included with the game. The "Ice Bird" acts like the Black Bird, but freezes everything around it instead. Makes the Blue Bird substantially more useful.
Rovio decided to become Valve 2.0 and make Space Eagles cost more than the game itself. Not to mention the fact that the Space Eagles are 10 times harder to use than the Mighty Eagle. Don't expect this to ever change.
NASA also decided to ride in on the coattails of this game since their budget has been reduced to nothing.