Regarding Rosalina, it's also important to remember that the character we're getting is called Rosalina and Luma. The Luma has a full move set of unique attacks all to itself. Lumas are a big part of the Galaxy series -- they're everywhere.
The Super Mario Galaxy series is certainly noteworthy enough to deserve a playable character (they're the most acclaimed games on the Wii), and Rosalina & Luma perfectly encapsulate everything about those games. They have the atmosphere/setting covered with their cosmic effects, and the Launch Star, Star Bits, and Gravitational Pull represent Galaxy's gameplay mechanics. Hell, Rosalina & Luma's general play style is representative of Galaxy 2's co-op mode.