It's been noted that this sequel's existence is owed to the large quantity of leftovers from the original, leaving a bunch of galaxies on the cutting room floor. Super Mario Galaxy 2 feels like these throwaway levels were picked up, dusted off, polished off, and packed away.Read on
Super Mario Galaxy 2 is a rarity in the Mario series: a direct sequel (see Super Mario Galaxy). Although SMG2 started as as spillover of too much content from the original, it appears to have shaped up as a game unto itself. It features tons of gameplay mechanics and innovations not present in the first Galaxy, from Yoshi and Luigi as playable characters1 to new power ups2, more challenge3, and of course, new galaxies.
SMG 2 will also focus more on gameplay and less on plot (indeed, Rosalina, who played a major role in the original storyline, seems to be altogether missing from this sequel; she only appears in ghost form to guide the player over tough spots) and the hub world has been dumped in favor of a stage map, a la Super Mario Bros. 3, Super Mario World, et al.